For security reasons, the idCAT Certificate can only be downloaded once .
If you saved the .p12 or .pfx file to a location you still have access to when you did the initial download, you can reinstall it. Unfortunately, if you do not have it, you will have to request a new one from an idCAT Registration Entity in person.
If you think you might have the original file, here's how to find it: Where is my idCAT Certificate?
If you do not have the original file, but you have it installed in a browser, here we explain how to export it so that you can install it on another device: How to export and import the idCAT Certificate?