You can use CoSMoS to easily edit the Skell credits.
Type in the current Skell credit > First Scan > Sell something > Enter new Skell value > Next Scan. Should be 5 or so value addresses to select from, it should be one of the ones from the bottom. Try each one of them, edit the value address. Done.
Are you talking about the credits you can earn ingame, or you have to buy with real money?