So, this code works fine in VB.NET:

Dim sModifiedAccountNumber_AsciiHex
Dim iByte As Byte = 0
Dim iIdx As Integer = 0
Dim strByte As String = String.Empty

sModifiedAccountNumber_AsciiHex = "FC13"
For iIdx = 1 To 3 Step 2

iByte = CByte("&H" & Mid$(sModifiedAccountNumber_AsciiHex, iIdx, 2))
If iByte >= 120 And iByte <= 127 Then
iByte = iByte Or &H80
strByte = Hex$(iByte)
Do While Len(strByte) < 2
strByte = "0" & strByte
Loop
Mid$(sModifiedAccountNumber_AsciiHex, iIdx, 2) = strByte
End If

Next

but, how would I convert this to C#?
I mean, I have majority of it however this always results in a format exception being thrown:

..
..
for (int iIdx = 1; iIdx <= 3; iIdx += 2)
{
iByte = byte.Parse(("&H" + modAccountNumberAsciiHex.Substring((iIdx - 1), 2)));