In the Android version of SkyDemon, there is an "Edit Route" menu item that opens a box where you can textually change details of the route.

Question: How do I access this function on the PC version of SkyDemon? As far as I know this function is the only way you can add an alternate airport after a route has been created. I am using SD 3.2.1.0 on the PC.

Problem: If I use this function on the Android version of SkyDemon with a multi-flight route, all flights except the currently selected one disappears. (Even if I only change the route part and not the departure/destination airports.) I am using SD 3.2.1.22629 on the Android.

There is no equivalent function on the PC version of SkyDemon. However that function isn't really for setting an alternate; to do that, right click on the relevant airfield and choose Set as Alternate from the menu that appears (or press and hold on Android/iOS).
Thanks for letting us know about the issue with other sectors being replaced when you use that function, however. That sounds like a bug and we will investigate.

Thanks!

It is a bit tricky that SkyDemon has slightly different menus etc. on different platforms.