CSE Valid Certificate for Signature - "unable to locate" - due to expiration.

Whenever I send an e-mail I am informed that CSE Valid Certificate for Signature - “unable to locate” - due to expiration. When I try and follow the steps to renew the certificate via your website I m informed that “this certificate already exists”. I want to renew the CSE certificate for my e-mail address.
How do I go about it?
Thank you