Choose a license on the pricing section: Single website (€49, updates and support for 6 months), Unlimited websites (€79, 1 year) or Lifetime and unlimited (€99). After checkout you receive an email with the download link and your license key. Both are always available under Login → Purchase History on this site, where you can also download the latest release at any time.
Updating from an older version or from the free plugin: deactivate the old plugin first, then upload and activate the new zip. Your templates, gift items, orders and voucher codes are kept. Deactivating never deletes data.