Hi -
If you don’t have Rhino 7, you’ll either have to ask a person with Rhino 7 to save the file in the Rhino 6 format or try the Rhino file converter that you can download here:
https://discourse.mcneel.com/t/rhino-3dm-file-version-converter/
Note that saving in a previous file format can lead to loss of data.
-wim