When the license expires, you cannot re-use it again in the activation wizard. The uploading license file UI will return the "License is expired" error.
This article provides details on how to renew a license subscription contract.
It is necessary to contact your reseller if you want to renew the license. If you cannot contact your reseller, you can find a new one in your zone and they should be able to address your request. If you have any issues renewing the license or it was impossible to contact a reseller, you can contact the GFI Sales Team. Please provide them with your current license number, the name of your organization/company, and your previous invoice (if applicable).
Once the license is renewed, you can register your Kerio Control installation (Register Product or Update license info options in Configuration Assistant).