Follow these steps if your remote fork goes out of sync with remote master:
Your local repository is a clone of your fork, so that is your origin. You need to pull from your remote repository and push to your fork. Assuming that you are on your master branch locally you can pull any changes from the upstream repository (the one that you forked from)
Git pull upstream master
Then once you have resolved any merge issues etc you can push the changes to your fork.
Git push origin master
This answer taken from StackOverflow saved the day!