The iPhone mail app has changed. I cannot figure out how to sign out or switch email accounts.
Go to Settings -> Passwords & Accounts.
Look for the Accounts section.
Click on the account you want to disable and press the switch to de-select mail:
If you want to delete the account scroll to the bottom and click Delete Account!
Go to mail.app and at the top you will see all your accounts under All Inboxes and you can switch accounts there. Tap on All Inboxes to see emails from all accounts.