It does not need to be changed. The part you know is given on the command line with a _ replacing the word you do not know. (It expects exactly 12, 15, 18, 21, or 24 arguments on the command line, in your case 14 words and _ for the 15th that you do not know.)
Attention: Never share the part you know here or somewhere else on the Internet. Even if you can’t crack it now, it might help others to crack it before you.
It is Python 3, so you need that. If you are on Linux, it should already be there. If you are on Windows, it seems to be pretty easy to install that lately: Python on Windows 10 for beginners | Microsoft Docs
You run it in a shell (some kind of terminal with some kind of shell on Linux, PowerShell on Windows):
$ python3 bruteforce-BIP39.py word01 word02 … word14 _
Is that enough to get you started?
(In theory, the script can handle more than one unknown word, but it will take very long and the possibilities become too many to check manually already for a second missing word. My script cannot handle if you are not sure about the order and also there, the possibilities become too many quite fast.)