Tap the “Settings” button on the iPhone. Tap the “Mail, Contacts, Calendars” option to choose it. Scroll through the list of email accounts you have loaded into the iPhone. Tap the name of the Gmail account you want to delete and choose the red “Delete Account” button at the bottom of the screen.
How do I delete a Gmail account from my iPhone 5?
To change your email address, open the iOS Settings, tap “Mail, Contacts, Calendars” and then select your Apple ID. You can then select your current email address or search for a new one.