Iphone 3gs on tmobile version update help?

Muffinslayer123 Posts: 1
edited August 2012 in iPhone Forum
i have an iphone that runs on tmobile and its on version 3.1.2. I want to update it to the latest version but i dont know how to safely do it. i really want to do this to get apps that run on the current version of firmware. I want to do it so it still runs on t mobile after and is still jailbroken ,Thanks