how do I change an email password on my iPhone 11 Pro?

Go to Settings>Mail>Accounts>click on the account in question. Once you open that, you will need to tap on the Account at the top of the page there. It will open the Settings area. Under Incoming Mail Server, you will see the Password area. If you are receiving an error, does it indicate receiving mail, or sending mail? If not, then you should be able to just change it there. If not, you will need to tap on the listing for SMTP (the sending server), click on the server in question, then change the password there. If that does not work, then the easiest method is to delete the mail account, power the iPhone off and back on again, then add the account back. You will only need to enter your email and password once.