Now suppose you don't use git, and you don't have a clue about how a patch works. Then this is the easy way to apply it.
You download the patch file, and you save it on your file system, e.g. as
You use the command line. Supposing you unpacked the archive in
cd ~/dev/CDNTaxReceipts-3.1 patch -p1 < ~/Downloads/39.diff
Here you are. You patched the extension.
If you use Windows, the hardest part will be finding a patch executable. On almost every other OS, patch will be available by default.