una soluzione fuori dalle solite righe potrebbe essere...
codice:
    Dim FileInput As String
    Dim FileOutput As String
    Dim Cont As Integer
    Dim NumByte As Long
    Const Riga = 5
    
    Open "TuoFileInput" For Input As #1
    FileInput = Input(LOF(1), 1)
    
    While Cont < Riga - 1
        NumByte = InStr(NumByte + 1, FileInput, vbCrLf)
        Cont = Cont + 1
    Wend
    
    FileOutput = Left(FileInput, NumByte - 1) & vbCrLf & "riga che vuoi inserire al posto della precedente" & Mid(FileInput, InStr(NumByte + 1, FileInput, vbCrLf))
    Close #1
    
    Open "TuoFileOutput" For Output As #1
    Print #1, FileOutput;
    Close #1

    Kill "TuoFileInput"
    Name "TuoFileOutput" As "TuoFileInput"