Originally Posted by wbrianw
I got an unlocked HTC One that was originally on AT&T's network, but now I use it on T-Mobile. The OTA updates won't work since I'm not on AT&T's network and the RUU file has not been released yet. I'm debating whether I should root the device, wait for the RUU file, or bug a friend to use their AT&T sim card to get the update. I'm currently running android 4.1.2 and sense 5 (software number 1.26.502.10). Also if the RUU file does come out can I use it from 4.1 to get to 4.4 or will there be issues?
The easiest way is to borrow your friend's sim card but that just keeps you locked into AT&T's network. Since, you are running on T-Mobile it may be better to change your phone to look like T-Mobile so you can get all of the benefits of T-Mobile including their OTA updates. You can use this guide Complete Flashing Guide | Rooting
to Unlock your bootloader, install custom recovery and root. Then I would use this guide
get S-OFF. Once you are S-OFF, then change your MID using this tool MID change without ENG HBOOT
then change the CID. You can use this guide nkk71's SuperGUIDE to returning 100% back to stock
to show the steps to install the latest T-Mobile RUU
. Once the RUU is installed you are set with a stock T-Mobile rom, recovery and S-OFF. Keep S-OFF as long as you can, don't return to S-ON!