Can you elaborate where exactly you are seeing the old email address? Is it included in your OpenPGP key in GPG Keychain? If so, you should be able to revoke the userID in question: This KB-article explains how to revoke a key or user ID.
Steve on 18 Dec, 2018 11:24 PM
This KB-article explains how to add more than one email address to an existing key. The email addresses in a key are called userIDs and you can add as many as you want. You can of course also create an additional key for the other email address.
Important note: Note that all email addresses you add to a single key can be connected to one identity. Depending on your use-case this might be wanted or should be avoided. Please consider that first and then proceed.
So whatever fits your needs best is the way to go. Let me know if you have more questions after going through the KB article.