How do I change a unit?

I read an article on the gaming website about changing creatures. I found the data.pak file, opened it with WinRAR, found the desired unit, opened the file with Notepad, made changes, clicked save, but a window pops up asking to save the file in .txt format and choose where to save it, i.e., it's like "save as." On another computer, when you close the file and make changes, it says that some path was not found. Please describe the entire procedure in detail; I really want to adjust the balance. So, the problem is this: after changing the file in the archive, it's not possible to simply make changes to it, and when you click the save button, a "save as" window pops up.