Function ChangeRow(RTF as RichTextBox,hZeile As Long,sText As String)
Dim sRows() As String
Dim y As Integer
sRows() = Split (RTF.Text,Vbcrlf)
sRows(hZeile) = sText
For y = 0 to ubound(sRows())
rtf.text = rtf..text & sText & vbcrlf
Next Y
'hier ggf. noch Array löschen
End Function

Call ChangeRow(RichTextBox1,1,"Zeile 1")

Manchmal verliert man, manchmal gewinnen die Anderen...