Private Sub SearchReplace()
    Application.Selection.Find.ClearFormatting()
    Application.Selection.Find.Text = "find me"

    Application.Selection.Find.Replacement.ClearFormatting()
    Application.Selection.Find.Replacement.Text = "Found"

    Application.Selection.Find.Execute(Replace:=Word.WdReplace.wdReplaceAll)
End Sub

private void SearchReplace() 
{ 
    object replaceAll = Word.WdReplace.wdReplaceAll; 

    Application.Selection.Find.ClearFormatting(); 
    Application.Selection.Find.Text = "find me"; 

    Application.Selection.Find.Replacement.ClearFormatting(); 
    Application.Selection.Find.Replacement.Text = "Found"; 

    Application.Selection.Find.Execute(
        ref missing, ref missing, ref missing, ref missing, ref missing, 
        ref missing, ref missing, ref missing, ref missing, ref missing,
        ref replaceAll, ref missing, ref missing, ref missing, ref missing);
}