Any way to update VARBINARY value on certain position?
You can use the INSERT function:
INSERT(String, StartingPosition, CharsToDelete, 'StringToInsert')

So if you wanted to replace one character at position 4 in the string abcdefghi, the expression would go like this:
INSERT('abcdefghi', 4, 1, 'x')

See for yourself in this demo.
The expression would only return a modified string. Use it in an assignment to actually write the result, where necessary.