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I know there is a solution for CMD at https://stackoverflow.com/questions/7244321/how-do-i-upda
The popular answer didn't work for me - I kept getting a "A branch with that name already exists" error. Per this issue I found these instructions that did work:
upstream/master
and click "Merge into master"To sync the master of your fork to the master of the original repository using GitHub Desktop:
Click on the 'current branch' tab and first select 'master' as the current branch (if it's not already selected).
Click on the 'fetch origin' button.
Click on the 'current branch' tab again and click the 'choose a branch to merge into master' button at the bottom.
*NOTE: Looking down this list, you will find 2 entries for every branch. Those that are prefixed with origin/ are the branches in your fork, and those prefixed with upstream/ are those in the original repository on GitHub.
Select upstream/master from this list, and this pull the changes down from the master repository to bring your local clone up to date.
Once you local clone has finished pulling the updates from the master repo, push these new changes to your fork, stored on GitHub, using the push origin button on GitHub desktop.