Today I wanted to make a PR against phpBB but I noticed my fork was missing a new branch, which I needed.
So I opened the terminal (Git Bash) on my PC, from the master branch of the local copy of my fork.
Set the upstream to the phpBB repository (upstream, the original)
git remote add upstream https://github.com/phpbb/phpbb.git
Fetch all there is
git fetch upstream
Set up to track remote branch ‘prep-release-3.3.1′ from ‘upstream’
git checkout prep-release-3.3.1
Push from local to my own phpBB fork (origin)
git push origin prep-release-3.3.1
git checkout prep-release-3.3.1 && git pull https://github.com/phpbb/phpbb.git prep-release-3.3.1 && git push origin prep-release-3.3.1
Now we are ready for the next.