Go to the PARAM-O webpage at https://www.graphisoft.com/param-o.
You have two options for installing PARAM-O.
Copy the apx file to the Add-Ons folder
Copy the apx file to your installed Archicad's "Add-Ons" folder. By default, Archicad is installed in the "Program Files" folder. After you copy the file, restart Archicad. PARAM-O will load automatically.
Load apx from a specific location
Place the apx file to any location in your computer. Start Archicad, then open Add-On Manager from the "Options/Add-On Manager" menu. In Add-On Manager, find the "Edit List of Available Add-Ons" section and click "Add". Choose the apx file and close Add-On Manager. Now PARAM-O is loaded.
Update the previous installation
To make the new version work, you must always remove the previously installed PARAM-O. If you placed it the previous version in the Add-Ons folder, then you have to delete it. If you added it with the Add-On Manager, you have to unload it.