A little example in updating your forked branch of UnrealEngine4 from Epic's repo into yours.

gistfile1.txt

Do the usual of getting your unrealengine and github account connected
create (or use your pre-existing ssh keys for github ) and add those keys to github
fork from https://github.com/EpicGames/UnrealEngine.git in github
clone from your fork (lets say for example mine git@github.com:salamanderrake/UnrealEngine.git)

>> Begin Commandline 

git clone git@github.com:salamanderrake/UnrealEngine.git
cd UnrealEngine
git remote add epic https://github.com/EpicGames/UnrealEngine.git
git fetch -n epic

>> End Commandline

Note 1: epic for remote name is just the one I use, you can call it what you want.
Note 2: -n in the fetch line is to not pull in tag data (I think, its been a while)

Epic does its thing and updates its release branch and you want those updates.
Make sure you are in the branch you want the changes pulled in from

>> Begin Commandline

git fetch -n epic
git pull epic release

>> End Commandline