How do I change the Gmail address on my iPhone?

Well, you have asked us to guess the context of your request. You told us how you’re trying to do something but not what you’re trying to accomplish.

So I will take a guess about your context which is very likely wrong because you’re trying to do something else.

If you’re trying to simply change your Gmail address for email (and not your Apple ID) and you are using Apple’s Mail app, go to:

Settings > Mail > Accounts > [your Gmail account] > Account.

Where do you want to change it? I can think of at least 4 places where it could be entered. Do you want the entire thesis? ;-)