How do I update my android to 5.1?

[AC Question]

My bf currently has a Sprint galaxy s3, that was flashed over to be used on Straight Talk. Right now its on android 4.1.2 Baseband L710VPUCMK3. Im not sure if it has custom recovery on it, or if it is rooted or not. Its on The People's Rom. He wants to update the android to 5.1, if that is possible. SO I wanted to know, which Rom do I use to get to 5.1? Can I flash a rom from what I am on now, to 5.1? Do I use Odin? I am kind of a NOOB, so please bare with me. Id appreciate any help and guidance. Thank You