how do I change de iCloud account on my device?

To change the iCloud ID you have to go to Settings>iCloud, tap Delete Account, provide the password for the old ID when prompted to turn off Find My iDevice, then sign back in with the ID you wish to use.

You may find when you do this that the password for the old ID isn't accepted, If this happens and if your old ID is an earlier version of your current ID, go to https://appleid.apple.com, click Manage my Apple ID and sign in with your current iCloud ID. Click edit next to the primary email account, change it back to your old email address and save the change. Then edit the name of the account to change it back to your old email address. You can now use your current password to turn off Find My iDevice on your device, even though it prompts you for the password for your old account ID. Then save any photo stream photos that you wish to keep to your camera roll. When finished go to Settings>iCloud, tap Delete Account and choose Delete from My iDevice when prompted (your iCloud data will still be in iCloud). Next, go back to https://appleid.apple.com and change your primary email address and iCloud ID name back to the way it was. Now you can go to Settings>iCloud and sign in with your current iCloud ID and password.