How to change a password on a email on my gmail

Probably you mean Apple ID. If that is so then...

Change your Apple ID password on your iPhone, iPad, iPod touch, or Apple Watch

1. Tap Settings > your name > Password & Security.
2. Tap Change Password.
3. 
4. Enter your current password or device passcode, then enter a new password and confirm the new password. Forgot your password?
5. Tap Change or Change Password.

Sisnesler Said:

"How to change a password on a email on my gmail: Why can’t I change my password on my email on my phone"

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Create and Use a New gmail App Password:

When setting up gmail in your Mail app, you need to create and use an app password. You would log in using that password, rather than your gmail password. Go Here: Sign in with App Passwords - Google Account Help - Google