Just go to Settings>General>Restrictions and enter the old passcode and turn off Restrictions. Then turn Restrictions back on and enter the new passcode.
If you forgot the Restrictions passcode
Forgotten RESTRICTIONS Passcode
- Restore from a backup that was make before you added the Restrictions passcode. If you restore from a backup made with the Restrictions passcode the Restrictions passcode is also restored. Place the iOS device in Recovery Mode if necessary to allow the restore.
- Restore to factory settings/new iPod. . Place the iOS device in Recovery Mode if necessary to allow the restore.