Here’s are GitHub’s “Merging via command line” instructions for pull requests (I am fulldecent, the other guy is ospr):
Step 1: From your project repository, check out a new branch and test the changes.
git checkout -b ospr-image-rendering master
git pull https://github.com/ospr/FDWaveformView.git image-rendering
Step 2: Merge the changes and update on GitHub.
git checkout master
git merge --no-ff ospr-image-rendering
git push origin master
Here is the additional step that sends your changes back upstream(?) to the PR originator.
git push https://github.com/ospr/FDWaveformView.git ospr-image-rendering:image-rendering