PRTG Traffic Grapher - User Manual
If Versions 1.x, 2.x, or 3.x are installed
There is no direct upgrade path for older installations (before version 4) to version 6. There is an article in the Paessler knowledge base that explains what you can do.
What is PRTG Traffic Grapher?
PRTG Traffic Grapher is an easy to use Windows software for monitoring and classifying bandwidth usage. It provides system administrators with live readings and long-term usage trends for their network devices. The most common usage is bandwidth usage monitoring, but you can also monitor many other aspects of your network like memory and CPU utilizations.