sprintf from stdio.h will help you.
Edit: Argh, sorry, it won't help you. Thought into the wrong direction
However, you could use Lite Foundation to create an array with the highscore and then serialize the array and save it into the file. Ist aber die Sprichwörtliche "Kanone auf Spatzen"

And you can read whatever you want, the suffix gives you just a pointer of what you might find inside the file. But often you need to write a parser for the specific file type