OK, the title may be a bit misleading.  But this is how you can try to do it:

For Project RVT files:

  1. Export from higher version of Revit to IFC
  2. Import the IFC file into the lower version of Revit

Tip – use the best available IFC format that both versions of Revit understand

For Family RFA files:

  1. Load the family into a project in the higher version
  2. Follow the above steps for Project files
  3. Once you have the project in the lower version of Revit, open the downgraded family from the Project (select in Project Browser – Edit)

or //

  1. Export to DWG or SAT from the higher version
  2. Import the file into the lower version

Tip – you probably can’t explode the resulting import, or everything will disappear.

Basic tip via CAD Forum – How to save Revit project to an older version?

Also check out Sharing a Revit File with a Previous Release