How to replace/correct document after sent using C# SDK EnvelopAPI?

I started with this code below, but soon found out this will not work after envelope has already sent out.

Document contractPdfDocument = new Document
{
    DocumentBase64 = contractPdf,               //contract document    
    Name = contractId.ToString() + "_" + versionNumber.ToString() + "_" + DateTime.Now.ToString("ddMMyyyyHHmmss"),
    FileExtension = "pdf",                      //file extension
    DocumentId = contractId.ToString()          //source document id
};
var updatedDocumentResult = envelopesApi.UpdateDocuments(
    GetAccountID(), envelopeId, new EnvelopeDefinition() { Documents = new List<Document> { contractPdfDocument } });