Instructions are from ios5 but still apply just change version numbers accordingly...
Step 1: Open up the iFile application on the device to gain access to the file structure. Alternatively, users can SSH into the jailbroken device using a program such as FileZilla or CyberDuck.
Step 2: Navigate to the following directory /System/Library/CoreServices.
Step 3: At the very bottom of the CoreServices directory there will be a file called SystemVersion.plist. Open up this file up by selecting it and then tapping on Text Viewer.
Step 4: At the bottom of the file is a line of text which reads <string>5.0.1</string>. Edit the file using iFile, or a standard text editing package and amend the 5.0.1′ to say 5.1′. Note: If you are running an earlier version of iOS, the version number contained within the string tags will reflect that version.