Update a binary file in ascii method in c#

I have some fixed hex values to be updated to a binary file. I read the binary file using a hex editor and i found that it needs an updation programattically.

using (var stream = new FileStream(AsciifileName, FileMode.Open,    FileAccess.ReadWrite))
    {
        stream.Position = i;
        stream.WriteByte(arrayval);
    }

I tried using this but i couldnot manage the write as it is showing mis alignment.

I have a list with hexa values like

27 58 30 34 30 31 32 02

i want it to update at 2nd position as :

27 66 30 34 30 31 32 02

Is there somebody who is familiarized with this issue?.

**Steps needed **

1. Convert an Binary to Ascii
2. Update the Ascii file with my ascii values
3. Convert back to binary