you can do it from the registry.
open regedit
go to HKEY_CLASSES_ROOT\Cubase.Document
find shell, then find open, then command
so basically find: HKEY_CLASSES_ROOT\Cubase.Document\shell\open\command
then, change the address to program files rather than program files (x86)
OF COURSE DO THIS IF YOU ARE COMFORTABLE WITH ALTERING YOUR REGISTRY! If you are inexperienced and change a wrong thing, it can lead to more problems…friendly warning 