During the collaborative development, it is easy to find out that the local branch of your own repo is out-dated with the branch in remote repo.
For example, there is a repo named remote/repo. Then you fork it within your own github as you/repo.
To keep the develop branch in you/repo with the develop in remote/repo, the following steps may be needed.
Checkout
Checkout two develop branches.
$ git checkout -b remote-develop remote/develop
$ git checkout -b develop you/develop
Merge
If there are commits after the lastest sync, the remote-develop will be ahead of develop with several changes. Then you need to keep the develop with remote-develop.
$ git checkout develop
$ git merge remote-develop
Then you have updated the local branch develop with lastest code. Finally the update for you/repo should be made to keep the two repos sync.
$ git push you develop