You would need to purchase a new owned license and then submit for the cancellation of your old monthly license. We unfortunately cannot perform an "upgrade" between the license types and the leased license would need to be returned to the licensing pool.

Once you purchase and receive your new owned license then you can just change the license key within the "Change License Key" section under the "Help" menu of your WHMCS admin area. You may alternatively just edit the license key within the configuration.php file to the new key which we provide.