Log onto your Plesk account.
On your Dashboard, click on 'Website & Domains' on the left-hand side of the webpage, unless you are already on it.
Under the Domain you would like to change details of the webmail for, click on 'Mail Settings'.
From here, you can choose an SSL/TLS certificate for your mail and/or webmail.
Once you have all the necessary details done, click 'Apply'.