This is on a 2017 Pixelbook.
I go to Settings, search for "Channel", click "Change channel", click "Stable" and "Change channel and Powerwash".
Once a download finishes, it reboots, I see a Powerwash screen, log back in and go through setup, and then I'm presented with "Beta 135.0.7049.19" in the bottom right menu. If I go to Settings, "Beta" is selected again.
I've gone through this whole process twice now.
The second time I first disconnected my Android phone, as I saw somewhere that someone had a similar issue and disconnecting their phone fixed it for them. But no dice.