I want to know which method should I call (and on which object) and how to call that method (required parameters and their meanings).
import github
g = github.Github(token)
# or g = github.Github(login, password)
repo = g.get_user().get_repo("repo_name")
file = repo.get_file_contents("/your_file.txt")
# update
repo.update_file("/your_file.txt", "your_commit_message", "your_new_file_content", file.sha)
If you are using token then you should have at least repo scope of your token to do it. https://developer.github.com/v3/oauth/#scopes
See: https://developer.github.com/v3/repos/contents/ and https://github.com/PyGithub/PyGithub