my iphone 7 has no place in settings to add my mail password. i cannot send mail and get an error message that I am using the wrong password. what to do?

When I tried to send an email from the mail app on my iPhone 7 (iOS 11), an error message said my password or user name is wrong and provided an option to goto Settings. In Settings (accounts & passwords), I see the username is correct, and there’s no place to add or change my password. How do I make sure this password is the correct one to login to my email account so I can access it from my iPhone?