You can change your SSL certificate's "common name," which is the primary domain name which it secures/encrypts.
Hosted... | Do this... |
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Shared hosting or Website Builder | Select Change to a hosted domain, and then select the domain name you want to use. |
Elsewhere | Deselect Change to a hosted domain. Enter the Common name you want to use for the certificate. |
You'll now need to verify the request for the new common name (more info).