Unfortunately we are just mostly community members here, so cannot do that for you. You would probably be better off opening a support ticket here: https://support.gitlab.com/hc/en-us/requests/new
If you can still login as that user, ask them to change/update it themselves, or perhaps if there is a access token, then it could be done via API as well. If not, then support ticket with Gitlab is only your chance to do it.