If you're using the Apple Mail app and you change your email password using the email provider's website, after doing so, you will need to enter the updated password for use with the Apple Mail App as well. This can be done using the following instructions.
- Open the Settings app on your iPhone or iPad.
- Scroll down, then tap "Passwords".
- Enter your iPhone's or iPad's passcode, or verify your identity with Face or Touch ID.
- Select your email account from your lists of sites.
- At this point, you should be able to see the saved email password in the "Password" field.
- Tap the "Edit" button on the top right corner of the screen.
- Enter the correct email password
- Tap "Done" in the upper right to save your changes.