using IronWord;

//  Abrir Word existente
WordDocument doc = new WordDocument("sample.docx");

//  Editar texto existente
doc.Paragraphs[0].Texts[0].Text = "This is the edited text.";

//  Exportar docx
doc.SaveAs("document.docx");

Imports IronWord

'  Abrir Word existente
Private doc As New WordDocument("sample.docx")

'  Editar texto existente
Private doc.Paragraphs(0).Texts(0).Text = "This is the edited text."

'  Exportar docx
doc.SaveAs("document.docx")