Download El Capitan Installer now that Sierra is out
It depends on your circumstances. It can be difficult to find old OS downloads.

If you purchased it previously it will be in the App Store under the Purchased tab. 

If your mac came with it pre-installed you may have to go the internet recovery or Recovery HD route, but that would entail wiping your hard drive in order to do the install. You could then migrate your user data from a Time Machine backup or similar. 

Alternatively, if you have a bootable backup of your El Cap system (assuming you had it installed) you could reinstall that. 

More specific answers will require more specific information from you (it's generally a good idea/helpful to provide more information so that answers can be geared towards your situation and others in the same situation can benefit). 

Whatever you do, make sure you have a full working backup (which you have tested) so that you can revert should something fail.