Codenstuff.com

        Dim fs As FileStream = New FileStream("test.exe", FileMode.Open)

        fs.Position = &HFFA0 'an example offset of the text

        Dim changed As String = "new text" 'must be equal or shorter than the original text
        ' ASCII encoding is assumed
        For i As Integer = 0 To changed.Length - 1
            fs.WriteByte(Asc(changed(i)))
        Next
        fs.Close()

        Dim input As Byte() = File.ReadAllBytes("file.txt")

        Dim lookFor As Byte() = Encoding.ASCII.GetBytes("old text")

        Dim temp(lookFor.Length - 1) As Byte

        Dim offset As Integer = -1
        For i As Integer = 0 To input.Length - lookFor.Length
            System.Buffer.BlockCopy(input, i, temp, 0, lookFor.Length)

            If temp.SequenceEqual(lookFor) Then
                Dim dr As DialogResult = MessageBox.Show("Found 'old text' at offset " & i & ", finish search?", "", MessageBoxButtons.YesNo)
                If dr = Windows.Forms.DialogResult.Yes Then
                    offset = i
                    Exit For
                End If
                'else keep looking
            End If
        Next