A valid license is required in order to update an add-on. Many add-ons that come without a license key can only be updated manually (e.g. Slider add-on).
If automatic updates are available, they will work like the updates WordPress provides for other plugins as well. You will see if a new version is available either on the Plugins page or on Dashboard > Updates.
Please check this page if automatic updates are not working.
You can also find the latest version of your add-ons through your account.