1. Clone the fork:
  `git clone <github url of the original repo>`

2. Add remote from original repository in your forked repository:
  `cd into/cloned/fork-repo`
  `git remote add upstream <git url of the original repo>`
  `git fetch upstream`

3. Updating your fork from original repo to keep up with their changes:
  `git pull upstream master`


Another way

1)`git fetch upstream`
Check out the master branch from your local fork.

2)`git checkout master`
Now merge the changes from upstream/master into your local master branch.
Your fork’s master branch will be in sync with the upstream repository. You will not lose your local changes.

3)`git merge upstream/master`