Unfortunately, there is no way to extend the expiration time of a document once it has become expired. Expiration dates are set at 60 days by default, however, with a Business or Enterprise plan, it is possible to set up your documents to auto-expire after up to 3650 days. This article explains how to set your own expiration date.
As a workaround, you can click Actions > Edit on the sent/expired document. It will turn it back into a draft and will allow you to edit the document and resend it. Please note, editing a document will erase signature and initials fields to protect the signer from having signed an altered document.
You can also copy the expired document. Here is the article on duplicating documents.