When it comes out sometime this fall, it will appear as a free update in the mac app store. You can choose to install it or not, it's up to you. If you do it that way and install it straight from the store using the install app, it will just upgrade right over your current Mavericks install and all your files and apps will still be available just as they were in Mavericks. If you want to do a clean format install you will have to turn the download into a bootable usb and then boot from the sub to install the OS. During this time you will choose your drive and format it and then begin the installation.