If you are using the AirPOS App Store version 3.8.18, and have previously setup your Zettle integration in your backoffice, you should first disable Zettle in Payments Settings, before updating your AppStore version to 3.8.21.
Under "Do you use Zettle?", click No
You will see a disconnection notice which will confirm when the integration is disconnected
The Setting will show "No" under "Do you use Zettle?"
You should now go to the App Store and update your AirPOS app to version 3.8.21
Once the update has finished, you should re-enable Zettle in the Payment Settings by clicking "Yes" under "Do you use Zettle?"
This time, no window will appear but Zettle will appear as a tender type during a transaction. When you make your first Zettle transaction after having enabled Zettle, you will see a Zettle login window
Enter your Zettle account credentials and you will be signed in
You will need to reconnect with your Zettle card reader