First you need to get the latest from upstream master into your fork master: https://help.github.com/articles/syncing-a-fork/

From there, you will want to rebase master into your branch (assuming your branch is just your work on this issue!!!). Checkout your branch, and run `git rebase master`. Essentially that will un-apply the work you did on your branch from the "branch point". It will then update your branch to apply the upstream master changes, and finally try to reapply your changes.