The update action can be done from two different places in the UI:
Navigate to the Domain Settings page from the Environments screen.
Alternatively, navigate to the specific Environment Details page from Environments screen and navigate to the Domain Names table.
Identify the row where the custom domain name you wish to view or update is listed.
Select the … menu from the far right end of the row
Select the View & Update option.
Make changes to the SSL certificate as necessary and confirm your submission.