update ios 6.1 ?

The iPhone 3GS can be upgraded to iOS 6.1.3.

If your iDevice is using a version of iOS lower than 5, you will need to use iTunes on your syncing computer to perform the upgrade. Use the Apple link below as a guide for the upgrade.

http://support.apple.com/kb/HT4972

Also read the instructions from the section entitled "Update your device using iTunes" at the link below.

http://support.apple.com/kb/HT4623

Information regarding transferring purchases from your iDevice to iTunes on your syncing computer can be found at the link below.

http://support.apple.com/kb/ht1848

The latest update ( and the last ) for the iPhone 3Gs is iOS 6.1.3.

You have iOS 4... See Here...

http://support.apple.com/kb/HT4972