Is there any way I can approve part of pull request changes by changed files?
I mean when I see all changed files in pull request, I want to approve only couple of them, add them to repo, and rest of changes mark as 'to improve'.
Pull request are atomic, you cannot accept few files in request. Either its all or nothing. If your pull request has multiple commits and you want few commits out it then it can be achieved by making a new branch out of PR and then cherry-picking to master.