I would guess it depends on what you want. Sprint has the 6.5 on their website. I don't know where, but it's on there somewhere. (Not a Sprint customer) But if you are looking for a custom ROM there are plenty. So I would guess the first thing you want to make sure is that you have unlocked the phone. The HSPL unlocker is
HERE. Follow the directions word for word. Once you unlock the phone, you then can flash a custom ROM. (With the Sprint stock 6.5 rom you don't need to unlock phone) If you want a custom rom the find one
here, the one I use is linked on the bottom picture. Hope this was some help, at least enough to point you in the right direction.