You would need to contact RealFlight Support and see if they can help. Without sufficient documentation, I'm not sure if they can, though.
Ideally, the previous registered owner would contact Support to unregister the software, clearing the way for you to then register it in your own name and perform an online update to the latest version.
Failing that, if the previous owner tells you their password, in theory you could register it with that same password but with your own information (name, address, etc.). Then the info on file would match what you could supply to Support, at which point maybe they could help reset the password for you so you could set it to something new of your own choosing by re-registering.