Scenery can be added or removed at any point in the future by re-running the installer. When the X-System installer comes up saying “You already have X‑Plane 11 installed on this computer,” click the Add or Remove Scenery button.
Select the scenery you would like to install. Parts of the world that are currently selected will be a bright blue color. Note that for regions where no scenery is installed, only oceans and airports will be visible. If you are unsure what areas are currently selected, just click Select None to turn everything off. From there, move your mouse around the map to highlight sections of the world and click on on the ones you would like to install.
You can read more about installation of X-Plane here.