How to change the password of an e-mail account depends on the e-mail app. What app do you use?
https://www.digitaltrends.com/mobile/best-email-apps-for-the-iphone/ mentions the "10 best", and we can't guess which one you use.
But it seems always possible to just delete the account and create a new one with the new password.
I have an mini ipad4 and I use my AOL email on it. I had to change my password for AOL and its changed on all devices except the ipad. How do I change it?