Update Iphone 3gs by ios 5.1.1...?

[ya.ali]

Hello Everyone

i want to update my iphone 3gs from ios 4.2.1 to 5.1.1.
my iphone information is following

Carrier:AIRTEL
CAPACITY:14.2GB(16GB)
VERSION:4.2.1(8C148a)
CARIAR:Carrier 9.0(VADAFONE LTD.)
Model:MC131B
SERIEAL:86952WV53NP
IMEI:01 216000 ?????? ?
FIRMWARE:06.15.00
COUNTRY:UNITED KINGDOM
SIMID:23415
iBoot: NewBootrom 359.3.2

Now question is...???
How to update my iphone 3gs by 5.1.1 without losing baseband (6.15.0) using redsnow.

Note: Phone is currently unlock and jailbroken.

thanks for any kind of help.

 

[stonyboys]

I'm not up to date with jailbreaking, but if you don't want to upgrade baseband because of unlock, there's no need to worry because now that your phone is unlocked, it's permanent no matter the baseband.