The email address is actually associated with your ACME account, so modifying it will cause expiration notices and such to be sent to the new email address for all certificates ever issued for that ACME account. You need to use the functionally of your ACME client to make this change. In your case, that's likely the interface panel of your DSM.
@griffin, thank you for the Info. I do keep the Synology DSM software up-to-date. Although I had previously read Certificate | DSM - Synology Knowledge Center info., I looked through it again. I do see the certificate in my Synology DSM software ( Control Panel > Security > Certificate). I do not want to "cause expiration notices and such to be sent to the new email address," so how do I get emails about my current Let's Encrypt certificate to go to a different email address?
I do not see such an option in the DMS software?
Am I understanding you correctly that such a change can only be made via the DMS software?
If not please provide some details about the how-to.
If so then I will contact Synology for assistance.