How to switch to windows operating system/ windows and mac??

I want to be able to have my game run on a windows, I've developed on a mac os High sierra, how can I make windows customers be able to use it?

I go onto the build settings and when I go on the drop down menu on the targeted platform it doesn't let me switch to windows the only operating system there is "Mac OS"

 

That means you didn't install the Windows build components. Run the Unity installer again, and include it this time.

And when it comes time for code-signing and packaging, you may be interested in my blog post on code-signing Windows apps on a Mac.

 

You should have a separate hard drive to run OS High Sierra, then run Unity installer and include the additional hard drive.