How to upgrade the iOS of Jailbroken iPhone 3GS with baseband 05.13.04

I have a iPhone 3GS which was a jailbroken (untethered) a year ago, now I want to upgrade the iOS to 5.x or higheras mine is now 4.1 and most of the new apps are not supporting the iOS devices which are having less than 4.3. Since my device is having old boot rom, someone from this forum advised me to downgrade the baseband to 05.13.04 before i make the iOS upgrade and i DID downgrade successfully. Now can you please guide me to update the iOS. Here are my device details Version: 4.1(8B117) Model: MB715LL Modem Firmware: 05.13.04 my previous question and answers http://www.iclarified.com/Q435/how-to-upgrade-the-ios-of-jailbroken-iphone-3gs