If it is a Quest game, then in the headset open your Apps tab. From there, there should be a drop down in the top right corner of the window. Select either All Apps or Uninstalled. You can then choose to install any apps you bought that aren't installed. If you bought them directly from within the headset, they should install automatically. If you bought them from the Rift section of the store though, you can't install onto your Quest unless it's cross-buy. In that case, go into your Quest, visit the store, and check that app's page. It should allow you to download the app without having to buy it a second time.
The reverse also works if you bought a game on your Quest that's cross-buy enabled. Open the Oculus app on your desktop, go to the game's store page, and download for free.