Was looking for the solution of a similar problem. Thanks to the answer provided by Davlet and Cupcake I was able to solve my problem.
Posting this answer here since I think this is the intended question
So I guess generally the problem that people like me face is what to do when a repo is cloned by another user on a server and that user is no longer associated with the repo.
How to pull from the repo without using the credentials of the old user ?
You edit the .git/config file of your repo.
and change
url = https://<old-username>@github.com/abc/repo.git/
to
url = https://<new-username>@github.com/abc/repo.git/
After saving the changes, from now onwards git pull will pull data while using credentials of the new user.
I hope this helps anyone with a similar problem