1. Clone your forked project

This is only important if it's not yet in your workspace

git clone git@github.com:YOUR-USERNAME/YOUR-FORKED-PROJECT.git

2. Add remote from original repository in your forked one

cd into/cloned/forked-project
git remote add upstream git://github.com/ORIGINAL-DEV-USERNAME/PROJECT-YOU-FORKED-FROM.git
git fetch upstream

3. Updating your forked project from the original one to have their changes

git pull upstream master

4. Solve conflicts (if any)

5. Finally push changes

git push