How to backup your files if you want to reformat your PC

Depends on what files you want to backup. Windows includes a backup utility that can backup files.

Why do you want to reformat?

If you're going to reformat your PC, I assume you mean your hard drive? In any case, you can find a backup utility under Control Panel (correct me if I'm wrong, I cba to check). If you're going to format your hard drive, you're going to need another hard drive or an external one.