If you still have the key, then the software is available for download from the official website. However, the upgrade is not free and users with a key for version 4 of Janus are instructed to enter it and select the ** Janus 5 Upgrade from Janus 4** form on the same website.
Access the page, enter the key and then you'll be able to upgrade for $79.00 USD. This is the only possible way. If you have any other questions about this process, get in touch with the developers.