1. Open the EAC
Click on start and then expand Exchange Server 2016 and click on Exchange Admin Center.
2. Login to the EAC
If you have a certificate warning, click yes to continue and then enter the credentials to login and press enter.
3. Click on Servers
On the left hand side, click on Servers.
4. Double click a server
On the general page, enter in your product key and click save.