Upgrade from iPhone 3GS iOS 4.3.5 BB 05.16.02 to iOS 5.1.1

Hi, I have an iPhone 3GS iOS 4.3.5 BB 05.16.02 (thethered new bootrom) and I want to upgrade to iOS 5.1.1 or newer, whatever it's fine. I don't want to upgrade my BB as I might loose my GPS or WIFI. Can anybody guide me trough the steps of doing this please? Thanks in advance. Akusmo