how to update i0s 4.1 .BB 2.10.04 to ios 5.1. or what ios i suppose to update

[luckytomyu]

HI, I need help , i just buy used iphone 4. with IOS 4.1(8B117) base band 2.10.04, what IOS i suppose to update and what base band have to update too?
I need to unlock too after that because now it AT&T , I'm going to used T Mobil sim
Thanks
Lucytomyu