https://arstechnica.com/gadgets/2021/10/google-ships-android-12-for-the-pixel-3-and-up/

Lost in the hubbub of yesterday's Pixel 6 launch is the fact that Android 12 shipped for older Pixel phones, too. Google took the odd step of releasing Android 12 to Google's AOSP (Android Open Source Project) repository on October 4, but it didn't ship any finished binaries for Pixel phones, which normally get day-one updates. During the Pixel 6 launch, Google's marketing department finally opened the floodgates with a blog post, a new website, a virtual AR statue, and builds for compatible devices.

Officially, Android 12 is out for the Pixel 5, 5a, 4, 4a, 3, and 3a. Android 12 marks the first major update the Pixel 2 has missed out on, though it has been without security updates for a while. You have a few options to get Android 12. The easiest is to wait for the update to hit your device, which will happen sometime in the next month as it automatically rolls out to users. Another option is to head to developer.google.com and download the OTA update for your device. There are also full from-scratch system images that will wipe out your existing data. Applying either of those last two will require some light command line work and tools downloads, and Google has listed full instructions on its site.