Folks who bought a Pixel 6a, the Android 13 Beta program is now open for you. Google added their newest phone to the program today, making factory image and OTA files available, as well as an opt-in through the official Android Beta page.
If you’d like to get Android 13 Beta up and running, the easiest way is going to be for you to hit the Android Beta Program site (here) and then click the “+ Opt in” button. Once you’ve done that, Google will push the update to your phone. After a few minutes, you can check for it by heading into Settings>System>Advanced>System update.
If you’d rather not wait for Google to push the update to you after joining the beta program, you can always flash a factory image or OTA file. Those are available here and here.
This Pixel 6a build of Android 13 is available as TPB4.220624.008.A2. This is an Android 13 Beta 4.1 build, in case you were curious. This beta brings the Pixel 6a in line with the rest of the Pixel devices in the program.