Open the Settings app
Tap Mail
Tap Accounts
Tap the email account you want to change
Tap Account
Edit the email address if the field is editable
If the email address cannot be edited, delete the account and add it again with the new email address
Go back to Accounts
Tap Add Account
Choose your email provider
Enter the new email address and password
Save the changes
Open the Mail app and confirm the new account is working