Update Iphone 3gs by ios 5.1.1...?

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.