If using a version of Revit prior to 2024, download the available installer file that matches the Unreal Engine version you are using, exit Revit, and run the installer file to install the exporter plugin. No download is required for Revit 2024 and higher. See the documentation for more details on installing (where applicable) the Revit Datasmith Exporter and using it with Unreal Engine. 

If you are installing these plugins on a computer that will not also have Unreal Engine installed, then you may first need to install some prerequisite dependencies for either 64-bit or 32-bit systems.