You would need to purchase a new monthly no-branding 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 license then you can just change the license key within the "Change License Key" section under the "Help" menu of your WHMCS admin area. The branding may not be removed automatically, in which case you should go to "Check for Updates" also under the "Help" menu.