Syncing your fork is always a good idea when you create a Pull Request.
Open your terminal and move to the local clone of your AzerothCore fork.
Switch to your master branch (or whatever branch you need to sync):
git checkout master
git remote add upstream https://github.com/azerothcore/azerothcore-wotlk.git
git fetch upstream
git merge upstream/master
Then when you are ready to push the changes (e.g. after resolving merge conflicts, if any):