From The Revit Clinic
If you have multiple versions of Revit on your system and you open files by double-clicking on them in Windows Explorer, you may find that Revit doesn't launch the version you want it to. If you have trouble with this, please try the following:
This example will set .RVT files to open with Revit Structure 2010.
Go to the Control Panel > Folder Options.
In the Folder Options dialog, click the File Types tab.
Scroll down to the RVT file type. In the bottom pane, click the Advanced button (if you've already made changes, click Restore, then the Advanced button will appear).
In the Edit File Type dialog, click New.
Under Action, type Open with Revit Structure 2010.
In the Application used to perform action field, type "C:\Program Files\Autodesk Revit Structure 2010\Program\Revit.exe" "%1".
Click OK to exit the dialogs.