If there's multiple entries like this in the file, you can't just delete arbitrary sections of the file. You need to read them into a data structure, remove what you want from the structure, and then write the structure back out to the file. If you have no idea where to start, learn about structs, linked lists, and then you should know all the basics to get this done.
edit: I see your original thread, and the answer is pretty much the same: The file is just for permanent storage - when you're manipulating data, you really need to have it in some kind of data structure. Also, this board has a strict No Homework policy. You're welcome to ask for help with homework questions, but when you do so you must show your attempt and ask specific questions.
