i have an iphone 3gs (Model No: A1303 FCC ID:BCGA1303A IC ID:579C-A1303A) and itune 10.7. But when i am trying to upgrade to ios5 it shows the message "my iphone software is 4.2.1 and is the current version". can anyone helpe me.

You do not have a 3GS, you have a 3G.

iOS 4.2.1 is the end of the road for a 3G.

Someone replaced the back of the case.

Yes if it will not update beyond iOS4.2.1 it is a 3G

or settings >general >about>model

if the phone says that is far as you can go with software then you have a 3g that is the proof because if you have a 3gs you could go to 6.0 so the model number means nothing at this point

FWIW those with that model believe they have a 3G ( simple google )