How do you change your Apple ID e-mail

On your iPad you can update it by tapping on your id i Settings > iTunes & App Store and then tapping 'view apple id' on the popup and logging into it, or you can do it via the Store > View Account menu option on your computer's iTunes or via http://appleid.apple.com

Changing primary email address : Change your Apple ID - Apple Support

After changing it : What to do after you change your Apple ID email address or password - Apple Support