Dim TextDatei As String="D:\irgendwas.txt"
Dim NeuerInhalt as string="Hier steht die neue Zeile"
Dim AlterInhalt As String = My.Computer.FileSystem.ReadAllText(TextDatei)
 
Dim sw As New System.Io.StreamReader(TextDatei)
sw.WriteLine(NeuerInhalt)
Sw.Write(AlterInhalt).
sw.Close

EALA FREYA FRESENA

Public Function InsertText(ByVal filepath As String, _
             ByVal texttoinsert As String, _
             Optional ByVal Index As Integer = 0) As Boolean
 
        If Not IO.File.Exists(filepath) Then Return False
        If String.IsNullOrWhiteSpace(texttoinsert) Then Return False
        If Index < 0 Then Index = 0
 
        Try
            Dim filetext As New System.Text.StringBuilder _
                (IO.File.ReadAllText(filepath))
            If Index > filetext.Length Then Index = filetext.Length
            filetext.Insert(Index, texttoinsert)
            IO.File.Delete(filepath)
            IO.File.WriteAllText(filepath, filetext.ToString)
            Return True
        Catch
            Return False
        End Try
End Function