If you are installing RelayFax for the first time, the software will generate a 30 day unrestricted trial licence for you to use - to continue using the product after your trial period has expired you will need to purchase a licence key which you then simply replace the trial licence key with - no further installation or configuration will be required.
To upgrade an existing installation simply download the latest release from above and install over the top of your current installation - all your current settings will be retained but it is good practice and highly recommended that you always make a back-up of your existing installation prior to upgrading!