I have an older PC that I have been using with Win 7 as its OS. It's still there on one of the three HDDs that's in the PC. I have since added a fourth drive, which is a WD Blue SSD which is 560 MB/s big, and has Windows 10 installed and running on it, when I start up this PC.

How do I go about replacing the Microsoft software with Firefox and Thunderbird? The Win 10 software is different than Win 7 so I don't know how to use very well. And the Win 10 instructions don't exactly match up with the Win 10 that is installed on my PC, so if some-body could list some of the steps that are more accurate than what MS has given, I would appreciate it.