According to TMONews, those sporting the T-Mobile branded version of the OnePlus 6T should be on the look out for an update containing software version A6013_34_181029. T-Mobile's support pages do not mention the update yet, but screenshots from owners of the device show that it weighs in at a modestly sized 132.3MB. The update contains some important improvements to several OnePlus 6T features and also includes the November Android security update.
The changelist from the screenshot notes that the update fine tunes Nightscape. This is the mode that OnePlus 6T users can enable when they want to snap photos under low-light conditions. The update also improves the in-display fingerprint scanner employed on the device, and comes with the latest versions of both the Unlock app and Gesture Mode 2.0.
While the unlocked version of the phone has already received several updates, the T-Mobile variant is expected to be updated on a slower pace due to certification differences. Since the update is being disseminated via an OTA update, it could take a few days to arrive. If you are not the patient type, you can try to manually receive the update by going to Settings > System > System update > Check for update.
Once the update is ready to be installed, you should make sure that you're on a Wi-Fi network with the phone's battery fully recharged before starting the updating process.