Alter e mail password

Have you already changed the password for you mail account? You probably would not get the prompt to change your passcode in the Mail client on the iPhone. Go to the web access for Yahoo and change the password there. Once you have done that, on the iPad and iPhone, you will need to go into Settings>Passwords & Accounts, then choose the Yahoo account. On the next page, tap on the account, and you should see the page to change the password for the client under Incoming Mail Server. Then tap on the Outgoing Mail Server listing that matches the Yahoo account. You may see another page listing Primary Server, then select the Yahoo server to get in and change the password there.

The only thing you can do from the phone is change them on the iPhone. That will not change them on the server. You have to do that from the web access of Yahoo, so you can get to the server settings.