If the server is not connected to the internet, you can manually update it by running the latest version installer. You can always get the latest version from this link. http://downloads.paessler.com/prtg/prtg.zip This direct update will only work if you are in at least version 18.x. If you are on an older version we recommend a stepped upgrade.
Once you have downloaded the latest version, you can install it directly over the version currently running. The configuration and database will not be affected by installing the newer version over the old and the configuration is automatically backed up before this process.
Keep in mind that you can only update if your Maintenance Service is still active. As updates are part of the Maintenance service, and your license will not activate on a PRTG version released after the Maintenance expiry date. In order to renew your maintenance you can do that here https://shop.paessler.com/shop/prtg/maintenance or you can contact our sales team at [email protected]