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 ~/Downloads/39.diff
You use the command line. Supposing you unpacked the archive in ~/dev/CDNTaxReceipts-3.1
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.