Go into DEP, click Manage Servers, and click on the tenant to renew.
If the Public Key has been renewed (This is likely and occurs annually) it will need to be uploaded. This is the APNS (MDM Certificate).
Click Replace Key…
Locate the current .pem file for the tenant and upload it as the Public Key.
Click Generate New Token… and click the Your Server Token option to download the token file.
Upload the token to the MDM by clicking on Choose File under Settings > Certificates > Apple/iOS certificates > Apple Device Enrollment Program (DEP) S/MIME token. Click Update at the top of the browser window.
If the above steps do not allow you to update your DEP certificate, please ensure that the following are true:
You are not attempting to assign the Apple DEP certificate to multiple MDM tenants.
The Mobility Suite's APNS (MDM) certificate is current and correct.