Minor updates and maintenance releases are free for users of the corresponding major version. In other words, if you purchased NoteTab v7.0, all updates up to v7.9 inclusive are available for free.
Major upgrades (version 8.x, 9.x, etc.) are available to registered users of previous versions at a 50% discount. Go to this link in order to receive your upgrade discount: http://www.notetab.com/upgrade
Should you have any trouble upgrading NoteTab or receiving your discount, please enquire with the helpdesk.