Installing 4.4.4 on SGH i337 running on TMobile Network - Need to upgrade to 5.0

[Travis Braden]

I am sorta a noob. Need help on where to get the 4.4.4 file for my ATT phone that is running on TMOBILE network. I am told I have to upgrade to this before upgrading to 5.0. If anyone has some advice on where to get the file and how to install I would be grateful!! Also, will there be any issues updating the phone since it is now running on TMOBILE?