The Samsung Galaxy Note20 Ultra 5G locked to AT&T's network is receiving the stable build of Android 11-based One UI 3.0.
The firmware carries version number N986USQU1CTL2 and requires a download of around 2.5GB. A complete changelog of the update is currently unavailable, but you can expect it to bring redesigned UI and new features to your Note20 Ultra 5G.
The One UI 3.0 update for AT&T's Note20 Ultra 5G is rolling out over the air and should reach all the units soon. But if you can't wait for it, you can try checking for the update manually by heading to your phone's Settings menu.