Q: how to change i cloud email

If you want to move your iCloud data to new iCloud account with a different ID, start by saving any photo stream photos to your camera roll (unless they are already there) by opening your my photo stream album, tapping Edit, tap the photos, tap Share, then tap Save to Camera Roll.  If you are syncing Notes with iCloud, you will need to open each one and email them to yourself so you can re-create the notes in your new account.

Then go to Settings>iCloud, scroll to the bottom and tap Delete Account.  (This will only delete the account from your phone, not from iCloud.  Your cousin's device will not be effected by this.)  When prompted about what to do with the iCloud data, select Keep On My iPhone.  Next, set up a new iCloud account using a different Apple ID (if you don't have one, tap Get a Free Apple ID at the bottom).  Then turn iCloud data syncing for contacts, etc. back to On, and when prompted about merging with iCloud, choose Merge.  This will upload the data to the new account.  Your old iCloud email address will remain with the original account and you will be assigned a new @icloud.com address when you turn Mail to On.

You will then have to go to icloud.com on your computer and sign into each iCloud account separately and manually delete the data you don't want from each account (such as deleting your cousin's data from your account, and your data from his account).