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

You should be able to install the .net framework 2.0 via your control panel; it will be somewhere as an option to turn on.

However, if you have previously been running SkyDemon, then you already have it.

As stated I am currently running Skydemon, so why the problem? But I will search in the Control Panel for clues.