A document was voided due to expiry date. How do I "unvoid" or reverse or extend the expiry?

You cannot "unvoid" a document, once voided you can only proceed to Send a new envelope with the correct document information.

In the future you can "Correct" an envelope to change the expiration date prior to it expiring, but again once voided, there is no alternatives to use the same envelope.

Here is a DocuSign article describing the void process and confirming that once voided there are no methods to reverse that change....https://support.docusign.com/en/guides/ndse-user-guide-void-documents