You can purchase a VPOP3 upgrade by going to http://www.pscs.co.uk/buynow/vpop3_upgrade.php

You will need to enter your current licence details, and the website will give you your upgrade options.

If you want to upgrade the licence size or from VPOP3 Basic to VPOP3 Enterprise at the same time as upgrading to the latest version of VPOP3, then choose the 'Upgrade to the latest version' option, and you will be given further choices of licence upgrades you can perform at the same time.