If you receive an envelope that should be Signed by another person use the "Assign to Someone Else" option under Actions in the upper right of the envelope after you open it. This does depend on whether the Sender "allowed" the envelope to be re-assigned. If the Sender did not allow Recipients to re-assign then the only option is to contact the Sender and have them correct the envelope to change the workflow and redirect the envelope to the proper Recipient.