AtT NOte 2 unlocked, now using Tmobile, Can't upgrade firmware

[jinjin12]

MY sister called ATt to unlock her phone when she went abroad, they did. Now she no longer has ATT and is with tmobile. Using the Tmobile sim card, she can't seem to upgrade her OS using the firmware update option in settings. How can she go about upgrading? she currently has android 4.1.2 with the NOte 2?

also if i flash aokp on her phone, but she don't like it, then flash back the original 4.1.2 firmware, would i lose the unlock that ATT did for her and she can't use Tmobile anymore?