Because your replacement is a different size than your input, you will need to create a new output file with the modifications; you will not be able to make the change "in place".
Open the input file, open the output file, loop 16 times using fgets() from input and fwrite() to output. fgets the 17th line and make the change to the string as required. Then fwrite() the new string to the output. Then loop over the rest of the file, fgets() and fwrite(). Finally fclose() both files.
