how to change icloud password

If you remember your current password then you can tap on your id in Settings > iCloud on your iPad and log into it, and on your account's screen there should be a Password & Security section where you can change the password. If using the same account for the stores you may have to log out of the account (tap on the id in Settings > iTunes & App Store) and back into it with the new password so as to 'refresh' the account for the stores.

If you can't remember your current password : http://iforgot.apple.com