See my reply in your other thread. As I tried to explain already, in C1Preview it is not recommended to use overlay to update the document's content after it's been created - it is a much better idea to simply change that content and re-generate the document.
BTW, a good approach to create documents where you may need to change/update some values later is to use tags. Simply put, tags are placeholders in the document content which can be replaced with actual values when the document is generated. Each tag is a key (name) - value pair, the tags collection is accessed via the C1PrintDocument.Tags collection, a tag may be referenced in the text of a RenderText or RenderParagraph object by including its name in TagOpenParen/TagCloseParen (square brackets by default). E.g.:
C1PrintDocument doc = new C1PrintDocument();
doc.Body.Children.Add(new RenderText("'mytag1' is [mytag1].");
...
doc.Tags.Add(new Tag("mytag1", "The text of mytag1"));
doc.Generate();
will produce the document with the text:
'mytag1' is The text of mytag1."
Attached is a small sample demonstrating this. Hope this helps.