To remove an old email from your iPhone

1. From a Home screen, navigate: Settings  > Passwords & Accounts.
    If unavailable, navigate: Settings > Accounts & Passwords.
2. From the ACCOUNTS section, tap an email account.
3. Tap Delete Account (at the bottom; may require scrolling).
4. 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.