3GS upgrade to ios5

[iphonestar]

Hi,

I have a US AT&T unlocked Iphone 3GS running on ios 3.1.3 and baseband 05.12.01. I would like to upgrade to the latest ios5 to take advantage of the icloud functionality. I am currently running my phone on O2 network and I don't have the original AT&T sim. Is it possible to do this currently or do I need to wait for some time ?

Thanks for your help !

 

[iPhoneklr]

i would contact your provider and ask their technician

 

[ovidiu1305]

Not a very good ideea to ask for the technicians opinion especially if is from at&t, because he will probably tell you to restore normally wich would update your baseband.
You are very lucky that you have an old baseband and you need to preserve that by updating using a custom firmware.You should update using a custom firmware that preserves baseband, I made one here:
iPhone Custom firmware - Downloads - Publisher - Unlock and Jailbreak iPhone 2G 3G 3GS 4G Online
Restore that, then jailbreak with the lattest redsn0w and unlock with ultrasn0w