To remove an old email from your iPhone
- From a Home screen, navigate: Settings > Passwords & Accounts.
If unavailable, navigate: Settings > Accounts & Passwords.
- From the ACCOUNTS section, tap an email account.
- Tap Delete Account (at the bottom; may require scrolling).
- To confirm, tap Delete from My iPhone.
To add the new one (For Mail 365)
Settings -> Passwords and Accounts
Choose Exchange then put in password and description
Then you will be forwarded to Microsoft. There put in the password and return.