After forking a repo, often you will want to keep it in sync with the upstream master repo. Here are the steps you can follow to accomplish that.
git clone git@github.com:YOUR_USERNAME/REPO_NAME.git
cd REPO_NAME
git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPO_NAME.git
git fetch upstream
3. Update your fork from the original repo to merge in their changes (replace master with proper branch name, if necessary)
git pull upstream master
4. Push your changes back to your github repo (again, replace master with proper branch name, if necessary)
git push origin master