- Transfer iTunes purchases to the computer by:
iTunes Store: Transferring purchases from your iOS device or iPod to a computer
- Transfer any other music by using a third-party program like one of those discussed here:
Copy music
- Connect the iPod to the computer and make a backup by right clicking on the iPod under Devices in iTunes and select Back Up
- Restore the iPod from that backup. The restore will also update the iOS.
Note that the backup that iTunes makes does not include synced media like apps and music