1. Launch the Exchange Admin Center
Click on start and then expand "Microsoft Exchange Server 2016" and click on Exchange Admin Center.
2. Login to the Exchange Admin Center
Type in your credentials and then sign into the EAC.
3. Click on the Servers tab on the left
On the left hand side, click Servers and then Certificates in the far right.
4. Click the + button
From the certificates tab, select the server from the drop-down list and then click the + button to create a new CSR. Follow the steps in the wizard"