First, import the certificate via MMC to the local computer > Personal store. After importing the certificate, go to 'Programs and Features' > Choose 'Windows Admin Center' and click 'Change'. It will open the installation wizard where you need to click 'Change' and can enter the thumbprint of the new certificate. https://windowsserver.uservoice.com/forums/295071-management-tools/suggestions/33950335-how-do-windows-admin-center-change-certificate

A brand new system management tool. I am looking forward to try it out. https://docs.microsoft.com/en-us/windows-server/manage/windows-admin-center/overview