With RichTextBox1
    .Text = "Sinn" & vbCrLf & "Unsinn" & vbCrLf & "Schwachsinn"
 
    Dim pos As Long
    pos = .Find("Unsinn", , , rtfWholeWord Or rtfMatchCase)
    If pos >= 0 Then
       .SelStart = pos
       .SelLength = Len("Unsinn")
       .SelText = "Blödsinn"
    End If
End With