F-Secure KEY Premium is an additional service in F-Secure KEY that allows you to connect your various devices and sync your KEY data between the devices. You can choose to buy the service for either 12 or 24 months. Once you've purchased the service, you are given a premium voucher code, which you can enter into KEY in the following way:
Open the F-Secure KEY app.
Go to Menu > Help > Enter Premium Voucher.
Enter the premium voucher code and click/tap Redeem.
After you have entered your premium voucher, you can sync your KEY devices. When synced, your KEY data is kept up to date on all devices automatically.