How do I modify “Signature” element properties, eg. "Id"?
First, you need to setup all properties needed for the signature, then call Sign() method. This method will prepare everything for signing and create “Signature” structure. Actual signing is performed in Save() method, because canonicalization results and digest values can depend on the signature element placement in the xml document.
Next, to change Signature element Id set ID property as shown (C# notation):
ElXMLSigner.Signature.ID = "id123";