Upgrading allows you to acquire license (and key) for a new major version of the product whose previous version license and key you have. Note that after upgrading you will still have the right to use the product, and you will still be possessing the previous version product key, but it will be changed to the "Not For Resale" status thus making further transfer of this product key impossible.
Upgrades are ordered at the User Area, via the "Upgrade" button placed near the product key for which an upgrade is available.
Upgrades are usually available with 50% discount off the regular price.
Crossgrades (if available) are handled similarly to upgrades.