Add a new remote pointing to the original repository:

$ git remote add upstream <original-repo.git>

Fetch the changes. Repeat this step whenever the original repository changes:

$ git fetch upstream

Update the local fork using merge:

$ git pull upstream master
$ git pull upstream <branch-to-update>

Or update the local fork using rebase:

$ git rebase upstream/master
$ git rebase upstream/<branch-to-rebase>