The OnePlus 6T has been running Android 10 for a few months, but not in T-Mobile. The units bought from the magenta operator have got stuck on Android 9 Pie, so it seemed like an eternity. But not anymore.
The operator finally began implementing the Android 10 update for its OnePlus 6T units today. As usual, this is an organized matter, so it may take a few days before your TMo 6T appears that update notification, but it is nevertheless underway.
Once you install the new ~ 1.6GB software, you will have Android 10 on board, as well as the security patch level of December 2019 …