You can change the iCloud account on your iPad by signing out of your old account and into a new one, or add a different account for app purchases.

You can change your iCloud account on an iPhone with a few taps, and you can also use a separate account for your iTunes & App Store purchases.

The importance of using strong, unique passwords for each account is emphasised. If your iCloud password is weak, it's time to upgrade it to something stronger with a mix of uppercase, lowercase, ...