I googled how to backup PRTG and I found two relevant articles, the first referring to the second:
https://kb.paessler.com/en/topic/523-how-do-i-back-up-all-data-and-configuration-of-my-prtg-installation refers to file locations in https://kb.paessler.com/en/topic/463-how-and-where-does-prtg-store-its-data
To be totally honest backing up PRTG does not appear to be straightforward at all ! And so much reading. I guess I could just back up the configuration file, but what happens during critical failure? We're all paranoid of a hard drive crash.
My suggestion is that the software has a built-in facility where you can select backup, and a basic schedule. Something like backup daily to FTP location. Maybe a tick box or two to select what needs to be backed up.
For now, after quite a bit of reading and a struggle, I found "ProgramData\Paessler\PRTG Network Monitor\PRTG Configuration.dat". The struggle was because "ProgramData" did not even appear in the root of the C: drive. I've saved it to the desktop. Let's hope the C: drive doesn't crash.