api to pull from official/master to local machine for authenticated user

I have the oauth2 access_token for the user with scope repo which means I have complete access to the public and private repos of the user. I have a repo in my local machine and I want to git pull (fetch and merge) from the official repo using the authenticated user’s access token. Is there a convenient api for that?

Is this really such a hard question that people can’t answer? Or am I not making sense? Atleast mention this so that I can rephrase my question.