And if you've already paid for it, then open the app and go to the top menu and in the Help menu you can choose to Check for Updates. If it's a valid license it will update to the current version. If it's not then it won't update and you'll need to pay to be able to update it further, depending on the type of license you purchased.
On a normal basis, paid version usually update automatically and you will usually see a popup in the bottom right corner telling you that a new version has been installed and that you need to restart the app.