No passwords and accounts option

The Accounts & Passwords is from an older iOS. If you have the newer iOS, to modify your mail accounts, go to Settings>Mail>Accounts>select the account in question and make your modification there. You will have to determine which is incorrect, the username or the password, but you can make your modifications there. If you are using calendars and notes from your mail accounts, while they will delete if you delete the mail account, they will return when you add the mail account back and turn on calendar and notes.

If you go to Settings>Mail>Accounts>select the account in question by tapping on the title, then on the next screen, tap on the Account name. Just below the Incoming Mail Server, you should see Host, Username, Password. You edit the password there. Just below that, you will see Outgoing Mail Server, SMTP, tap on that. The next page will list your primary and any other SMTP servers. Tap on the Primary if the Yahoo account is the primary. The next page will list Outgoing Mail Server, Host, Username, Password, use SSL, Authentication, Server port. Go back to Password, edit there.

If you are not seeing that, then it has something to do with how Yahoo mail is configured. But as I said originally, and supported by Idris, delete the account and add it back. That gives it the opportunity to verify the account settings when you enter it, and you will not experience the username and password issues.

Deleting the accounts and adding them back will also download a fresh set of settings for the account (the stuff you don't have to type in anymore because it's all done behind the scenes). That may also resolve any niggling issues.

Delete the accounts and add them back. When you add the back, all the data (e.g. calendars, email, contacts) will come right back.