What do you mean by "renaming the contents of the files"?

A file has a name and has content. The rename system call changes the name, not the content.

If you want to change the content, you will need to rewrite the file using open and print.

Please go back to step one and describe in short, simple English sentences what you have, and what you want, and what steps you think you need to take to get from the one place to the other.