Changing account details inside app

Hey everyone.. The is an app is called nbde mastery 50 bucks worth i've downloaded on my iphone and ipad by another person account.. And works fine.. Was planing to update to ios 7 so i have to back up my ipad and it shall remove any unauthorized app I backed up the app through ifunbox and itools and then i installed it but it asks me for account password now and i dont have it.. So im figuring out something CRAZY i want to change the account needed to be mine and i shall enter the password to work :D any way to do that i can access files through ifunbox but cant find it