How do I update someone else's fork?

I’m on the RailsBridge Bridge Troll project at GitHub - railsbridge/bridge_troll: Event management system for all the Bridges (RailsBridge, MobileBridge, GoBridge, etc!). I am NOT a collaborator. My fork is at GitHub - jhsu802701/bridge_troll: Go to the URL shown here for instructions on how to get started in MINUTES, not hours!.

Someone else submitted a pull request that is now lagging several commits behind the upstream master branch. This pull request is at Add footer with links. by domlet · Pull Request #580 · railsbridge/bridge_troll · GitHub.

How do I sync up the branch behind this pull request?

Ask the submitter to pull master back into their branch, and push again. That will bring it back together, and will also put the burden of resolving any conflicts between their branch and the rest of the project.