How to open latest version revit file in lower version? If there is no chance for approaching the client again to get their help !

Open the file using latest verion on net & save on lover version.

some people  save the file in IFC. form and then they import the file to Revit, for me thats not helpfull acctually you can't get the full components if you follow the previous way, the best solution you should work on the latest version, and better if you start your project in specific version 2014 or 2015 whatever better complete your work in same version .

you can try to save it as an .IFC file and then imported in revit.