Here are the steps from RevitForum (thanks Menschmal).
Instructions to set the correct revit.exe file as default for opening Revit files from the file browser in Windows 7
(e.g. I am testing Revit Architecture 2012, but I want 2011 to be my default for now)
Open the Registry Editor (type regedit in the command line in Windows). If you are concerned about messing something up, back up the registry before making any changes.
In the Registry Editor browse the tree to the branch below:
In the right pane you will see:
Data: "C:\Program Files\Autodesk\Revit Architecture 2011\Program\Revit.exe" /dde
Double-click on (Default) and the Edit String dialog box will open.
Edit the Value data field for the correct location for the version of Revit you want to use. For example, I changed 2012 to 2011.
Close the Registry Editor and you are good to go. This change seems to be the universal default for all extensions that open with the Revit.Project application (.rvt, .rfa, etc.).
Thanks to Menschmal at: