How to sync upsteam to downsteam fork?

[Gary Poon (Defond)]

I have forked a repository. And then I created a new branch in the fork to start coding for adding some features. After that the upsteam has made some changes on the original branch, I would like to sync them to the downsteam forked. 

I have read several online article but still cannot find out how to sync the upsteam to my downsteam fork.

Keeping forks synchronized and No option to manually sync fork said I should enable the auto sync option, but where can I found this option?