Let's say that when I installed Visual Studio 2017 I decided to use the "Windows Universal Platform" packages. Later, I realize that I'd like to install the ".Net Desktop packages" (either instead, or in addition).
How can I go back and add those features to Visual Studio 2017? Is there a way to do this within Visual Studio? Do I have to re-run the installer?
You can go to tools from menu bar and then select "Get tools and features" and then you can add or modify your features.
Re-run the visual studio launcher and click on modify. Make your edits and the launcher install the missing components.
After you have installed Visual Studio 2017 you will have a program called Visual Studio Installer. Launch this application.
From here you can select modify and then add the components that you need.