I am running SkyDemon on a Windows 10 laptop - currently running version 3.6.1.0. On trying to upgrade I get the message SkyDemon required Microsoft.NET Framework 2.0. I followed your (not obvious to find) link to a download page where I am offered three (operating system dependent) download options. Please advise:
Whether it is right that my Windows 10 laptop should require this download, and
Which of the three options to choose

I have no idea. If you are already running SkyDemon, you can run the latest version. I have never seen somebody's computer suddenly decide it doesn't have the .net framework prerequisite when it actually does.