Mac OS X 10.11 El Capitan is now available free from the Mac App Store. Anyone who has purchased Mac OS X 10.6 Snow Leopard, OS X 10.7 Lion, or OS X 10.8 Mountain Lion, or has OS X 10.9 Mavericks or OS X 10.10 Yosemite will be able to upgrade for no cost.
With this release, Apple has improved the security of the world’s most advanced Operating System by introducing SIP (System Integrity Protection), and some other improvements for a better user experience.
Note : not all bootloaders are able to do that:
If you are using Clover, Enoch, Ozmosis or RevoBoot probably you will able to do this (ensure you have latest version), otherwise ask the developpers of your bootloader if you can do that.