Someone declined to sign by accident. How do I reverse and resend?

You cannot reverse a decline as it "voids" the envelope and that cannot be undone. However, you can locate the envelope that was voided and select "Create a Copy" (right side dropdown) which will provide a new envelope with all the same documents, Recipients, email subject/body, and fields of the original so that you can send fairly quickly and without much hassle.

How do I create and copy ​

Locate the original envelope that you voided, likely under the Manage , "Sent" folder and go to the right dropdown and you should see "Create a Copy" as an option.

Will the signatures of people who have already signed it be there when you copy?

@Rahul Mourya​ When you void an envelope and use copy to create a new envelope from the original, the copy is going to have the Recipients, email text, and fields but it does not copy the previous signatures, those all need to be signed again by the Recipients.