(The subject of this question is random access to files,
in particular fopen(...,"r+") and fseek().
This subject will be deferred until after the second midterm.
However, sscanf(), which is used in the solution,
may appear on the second midterm.)

Write a changebyte program so that

   changebyte f p b

stores byte number b at position p in the file named f.
For example,

   changebyte data.txt 20 65

will change the 21st byte of data.txt to 65 ('A'),
leaving everything else in data.txt alone.

Do not read the previous contents of data.txt into memory.
Your program must work even if data.txt is too large to fit into memory.

If there are any problems, print a message on stderr and exit.