Hello my name is ***** ***** certified computer professional with over 10 years experience. I will be happy to help you.
Was this a user account on a specific website? If so what website? Also I am not completely clear on what you mean about a username on a page.
If you are saying that you created an account on some website which then was indexed in google and you would like it removed you do not have a lot of options. You can request the link be removed since it is outdated (because you changed the name) however if Google does see that request as valid it can still take weeks before the removal will happen to the the frequency of how often google updates the results of that specific page. The best way is to delete the page entirely. If you cannot do this yourself ask the person who runs the website to delete it.
In short, to remove a page from google you either need to delete the page, or request it's removal. Google will never just remove something because you do not want it there unless it contains personal information such as your social security number or something like that. I would suggest you try and get the page deleted. Even if it is deleted you may still see it in the search for weeks after until they update.
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