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.

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

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

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