Clone your fork.
```
git clone git@github.com:YOUR_USERNAME/FORKED_REPO.git
```

Add the remote upstream to git
```
git remote add upstream git://github.com/ORIGIN_USERNAME/ORIGIN_REPO.git
```

Fetch changes from upstream
```
git fetch upstream
```

Pull changes from one of the upstream's branches
```
git pull upstream BRANCH_NAME
```