There is an update option, which is free. On the menu line, click on Help, Online Update. Upgrading to a new version means buying a new version. For registered users, Magix send out emails with discounted offerings.
Upgrades are actually new installations. Sometimes you may have created some effects that you want to copy over to the new version. Sometimes Magix may have dropped some effects that you may want to port to the new version. You can keep the previous version installed for legacy reasons and uninstall it a couple of years later if it is no longer useful.
You have to accept to buy from the email which will give you a discounted version, or buy the new version from the website when it is on sale. The previous versions are never removed or "upgraded." A new version is installed completely separate from the previous version, even if you received an upgrade price. Even if there was a button, it probably wouldn't offer you a discounted price, so it is always better to wait for the email deal.
As I indicated, there is an Online update choice under Help, or by pressing Ctrl+Shift+F12. If an update is available it will indicate it and ask if you want to update.