Okay, not that I'm overly obsessing over every minute update, but I do like getting updates.
This one had me waiting a bit and I couldn't quite understand how to get it.
Simply put, it's patience. Open up your case and take the buds out. Just leave it.
I kept checking by opening the settings (bluetooth, and click the gear to see the Pixel Buds settings) and then in More Settings / Firmware update it would say there was an update available.
For the longest time it kept saying they were 552...
But eventually there they were at 553. I did put them back in the case for a bit and charged the case as I noticed it was low. Took them back out and saw it was now 553.
What the update was? No clue, Google's never really been that big on enforcing their own rules of posting changelogs.
I'm sure that had I not heard there was an update it might have happened all behind the scenes without any intervention of my own, but I like to be on top of things!