Love VonC's answer. Here's an easy version of it for beginners.
There is a git remote called origin which I am sure you are all aware of. Basically, you can add as many remotes to a git repo as you want. So, what we can do is introduce a new remote which is the original repo not the fork. I like to call it original
Let's add original repo's to our fork as a remote.
git remote add original https://git-repo/original/original.git
Now let's fetch the original repo to make sure we have the latest coded
git fetch original
As, VonC suggested, make sure we are on the master.
git checkout master
Now to bring our fork up to speed with the latest code on original repo, all we have to do is hard reset our master branch in accordance with the original remote.
git reset --hard original/master
And you are done :)