I am sorry, we only provide the current version for a download. If you have an old 188.8.131.5285 installation, please have a look in its installation directory, subfolder "download" to find the old installer executable.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.