code: public SignatureRequest updateSignatureRequest(String signatureRequestId, String signatureId,
              String newEmailAddress) throws HelloSignException {
          String url = BASE_URI + SIGNATURE_REQUEST_UPDATE_URI + "/" + signatureRequestId;
          return new SignatureRequest(httpClient.withAuth(auth).withPostField(Signature.SIGNATURE_ID, signatureId)
                  .withPostField(SignatureRequest.SIGREQ_SIGNER_EMAIL, newEmailAddress).post(url).asJson());
      }
docs: Update a signer's email address.
  
  This method requires the ID of the siganture request that has already
  been sent, as well as the signature_id that represents the signer that
  should be updated. The ema
  
  @param signatureRequestId String ID of the signature request that has
  already been sent and needs to be updated.
  @param signatureId String ID of the signer that needs to be updated.
  @param newEmailAddress String email address that the signer should be
  changed to
  @return SignatureRequest The updated request data
  @throws HelloSignException thrown if there's a problem processing the
  HTTP request or the JSON response.