PRTG Manual: Download PRTG
You can find the PRTG installer on the Paessler website. There, you can also log in to the Paessler shop to buy or upgrade your license or to renew your maintenance.
This only applies to PRTG on premises installations, not to PRTG hosted by Paessler instances.
Once you have installed PRTG, the auto-update automatically downloads and installs new software versions.