Renew Your Passport Online

You can renew your passport in three ways:

You can go in person. Many Post Offices and county clerk offices provide the service. You will likely have to make an appointment and be prepared for a wait.

You can fill out a form, get a passport photo, and mail it in, along with your existing passport. You certainly would want to use certified mail.

But a recent development is that you can do the whole process online. No appointment. No hand filling out of forms. No trip to get a passport photo. (Well, possibly no trip to get a passport photo. . .) You keep the old passport.
I will describe the online process in this article.

Renewing Online

Here are the requirements for renewing online:
The passport you are renewing is or was valid for 10 years, and you are age 25 or older.
The passport you are renewing is either expiring within one year or has expired less than five years ago.
You are not changing your name, sex, date of birth, or place of birth.
You are not traveling for at least 6 weeks from the date you will submit your application. Otherwise you would renew by mail.
You are applying for a regular (tourist) passport.
You live in the United States (either state or territory).
You have your passport with you, and it is not damaged or mutilated, and you have not reported it as lost or stolen.
You can pay for your passport using a credit or debit card.
You are able to upload a digital passport photo.

Want to renew by mail? Here's some info.

For your reference, here are the requirements if you renew by mail. It's a bit different since you will be mailing your existing passport.
You are willing to submit your current passport with the application.
Your existing passport:
Is not damaged (other than normal wear and tear),
Has never been reported lost or stolen,
Was issued within the last 15 years,
Was issued when you were age 16 or older, and was issued in your current name (or you can provide a document like a marriage certificate or divorce decree to show you have changed your name)

I included the additional information about renewing-by-mail because of some confusion/concern when I was researching the best way to renew. All three renewal processes use form "DS-82". So I assumed that (for mail-in or in-person) you would download DS-82 and fill it out by hand.
But the 'renew by mail' section on travel.state.gov has a "form filler" which allows you to fill it out online and then download it for mailing (or taking to an in-person renewal location). That sounds great. "Here's a nicely filled out form with no errors due to sloppy handwriting", I thought, especially when I read the instructions on how to fill out the form manually:

Use black INK.

NO mistakes. If you make a mistake, you must start a new form.

But you don't have to login when using the "form filler"! You can just go to "travel.state.gov", then the "Renew by Mail" section, and then begin to fill out form DS-82 - with your name, address, all your passport info, and your Social Security Number. In other words, just about all your key personal information!
I checked and "travel.state.gov" is indeed an official U.S. government site. And I checked further and the site itself explained that all your input is 128-bit encrypted, and is deleted after you finish.

So it's your call. But if you don't quite trust them, you can download the DS-82 form from https://eforms.state.gov/Forms/ds82_pdf.pdf. You can then print it and fill it out with the black ink.
And if you don't trust me, simply search for "Download ds-82 passport form".

So here's the steps to renew your passport online

Make sure your cell phone is handy. You will probably be sent a verification code. For setting up your account the first time, you may also need to have easy access to your email. A code might be sent to your email as well.
Start at travel.state.gov
Note that this site, along with Social Security and likely TSA, all use a common login area. It's called "login.gov". (You will be redirected there.) Create an account (or log in to your existing login.gov account). Thus you only need one username and password for the passport site and other government sites.
And who says the government is inefficient, Elon!

After you return from login.gov, here's the starting page for online passport renewal

Before proceeding with the entry of information, you will be shown the requirements page one more time.
On the next pages, you fill in your name, address, Social Security Number, and other information. Then you will enter your current passport number and date of issue.

Now comes the photo. (sigh)
You will upload a digital photo, in "JPG" format.
The whole process would have been easy except for the stringent requirements of the photo. You have to take a photo against a white or off white background. That's easy, you might say, except for the sadistic kicker: there can be no texture on this background! So that eliminates your patterned sheetrock walls!
You can perhaps buy a very large piece of white poster cardboard, or two medium size pieces abutting each other. You might be able to tape a white thin sheet against the wall and somehow smooth it out.
Note that at least you don't have to worry about how much extra area to include around your face. The renewal site will do the cropping.

Or, you can go somewhere and have a passport photo made. A county clerk office can usually do this. Most places can email you a "JPG" of the picture. (in addition to the physical photo itself). Make sure the photo shows a fairly white background! The photo that I received was underexposed a bit, and the background was actually grey, like the second picture below. (Apparently physical passport photos have a less stringent requirement for the lightness of the background.)
So it was rejected by the renewal site! I was actually able to lighten up the photo a bit using some software and resubmitted it, and they accepted ithe photo.

In this screenshot, I show that my attempted photo upload was "Inspected, Detected, and Rejected", in the words of that old Arlo Guthrie song. As you can see, the background was grey, apparently due to underexposure. However, I was able to put the photo into a photo/image editor and lighten it up, despite the warning to NOT do so (See the previous image). However I noticed in looking more closely at this screenshot, the renewal site actually offers an option to let the site itself attempt to fix it. Right below the failed image, it said "You can try to manually adjust your photo", with a clickable link. And right below that line was "Upload a new photo".

When you pass the photo gauntlet, then you "sign" the online form by entering the "ICN" number from the back cover of your passport. In my case, this actually matched the passport number itself.

Here's the final result.
As of March, 2025, the fee was $130 for the renewal. I obtained the photo from a nearby county clerks office for $30.

NOTE: I received an email from the U.S. Department of State the very next day, indicating that my payment was accepted and that the renewal process was underway, with a typical 4 - 6 week wait time. The email also indicated that my current passport is now cancelled.