BIM applications, including Autodesk Revit, use file formats, which are only upwards compatible - so by default you cannot save ("downgrade") projects to an older version of the given CAD/BIM application.
An alternative way of transferring a substantial part of your project (BIM model) to a previous version of the program is the exchange format IFC (Industry Foundation Classes, specified by IAI). Revit supports both export and import of this format. So instead of saving RVT perform an Export to a .IFC format file in the higher version, and then Import this file in the lower version. See "R" > Export > IFC and "R" > Open > IFC.
Always use the highest version (specification) of the IFC format supported in both versions of the application - e.g. "IFC 2x3" or "IFC 2x2" and set the requested export options (views, segmentation of building elements, etc.) - see "IFC Options".
Remember that part of the project information will not be transferred when using this method.