When you are ready to send forms out for completion, you will create and send a DocuSign Envelope. If the Envelope you sent is already marked as Completed, then no additional changes can be made to the Envelope. You will need to void the old Envelope, then send a new Envelope to the client to fill out with the correct information.
If the Envelope is still in process and waiting for others to take action, you could potentially correct the Envelope. There is a Collaboration option, if you want a recipient to make the edits. When adding recipients to an Envelope, check the "Recipients Can Collaborate" check box and resend the envelope. When the client gets the request to sign, they will be able to modify their previous answers.