The word "updated" in your search suggests you have an old copy of RF6.5 and want to patch it to the latest build (e.g., 6.50.026) or upgrade it to a newer version like Evolution. However, because RealFlight 6.5 is now over a decade old, its update servers are mostly offline. Therein lies the problem.
If the software is already installed, you can often see your serials in the upper right corner of the RealFlight Launcher screen. 2. How to Update Your Software real flight 65 serial number updated