how to use Word Object Model to programmatically replace text in a Word doc.

My function needs to read an MS Word doc, replace some text, and then SAVE AS PDF. Reading and saving PDF works great. But I can't figure out how to replace text by using Word Object Model.
Here is my word_to_PDF() function which does it all.
    Function word_to_PDF(Word_source_filepath, PDF_destin_filepath)
 Dim wordApplication As ApplicationClass = New ApplicationClass()  Dim wordDocument As Document = Nothing
 Dim paramExportFilePath As String = PDF_destin_filepath  Dim paramExportFormat As WdExportFormat = _   WdExportFormat.wdExportFormatPDF  Dim paramOpenAfterExport As Boolean = False  Dim paramExportOptimizeFor As WdExportOptimizeFor = _   WdExportOptimizeFor.wdExportOptimizeForPrint  Dim paramExportRange As WdExportRange = _   WdExportRange.wdExportAllDocument  Dim paramStartPage As Int32 = 0  Dim paramEndPage As Int32 = 0  Dim paramExportItem A ...