Link local project to existing GitHub repository

Originally I forked an existing GitHub project to my own account, made a few minor changes using Notepad++ and then uploaded those changes to the repository manually. As I started making more and more changes I started using pyCharm for development, and a local Git repository for version control, but still manually uploading the changes to GitHub. But this has become a PITA. I want to link my local project to my existing GitHub repository so I can just use pyCharm's built in upload functions but I can't seem to find a way to do that. It has a feature to clone a project from GitHub and a way to create a new project on GitHub from my source, but I don't see any way to link my existing project to an existing repo on GitHub. Is that possible? If so how?

I can do it outside pyCharm using CLI if need be, as long as once I'm done I can use the upload feature in pyCharm to commit changes. But I'm not all that familiar with Git so if you could provide some guidance that would be appreciated.

Thanks,
Dan