Unfortunately, there is not a way to extend the expiration time of a document once it has become expired. Expiration dates are set at 60 days, 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 "edit" on the sent/expired document. It will turn it back into a draft and allow you to edit and send again.
You can also copy the expired document. Here is the article on cloning documents.