Downgrade from El Capitan to Mavericks
If your Mac originally came with OS X Mavericks installed and you do not have a separate USB Installer for OS X Mavericks or never downloaded a complete OS X Mavericks Installer from the App Store, from which you can create a USB OS X Mavericks Installer, then you'd have to wipe the HDD/SSD  so the Recovery HD was not there and then start the Mac using OS X Internet Recovery (Command-R). Without the aforementioned USB Installer then OS X Internet Recovery is the only way to get what originally shipped on your Mac back.

Note: If in the past you had downloaded the full Installer for OS X Mavericks from the App Store but no longer have it you can still download it again from the App Store under Purchased and then create a USB Installer from it.

Be sure you are properly backed up before proceeding to reinstall, by whatever means.