How do I uninstall OS X sierra and go back to el capitan

Sure! First make a TimeMachine backup. Keep in mind some apps may not work under El Capitan if you've updated them since updating to Sierra.

Go to your Application folder to see if you still have the El Capitan installer. If not go to the Apps Store and then go to the Purchased column you'll need to find the El Capitan installer and download it again. It will launch you don't want to run it quite yet! Exit the installer.

Now the hard part you'll need to create a USB thumb drive installer follow this guide: How to make a bootable OS X 10.11 El Capitan installer drive. Once you create it reboot your system and holding the Option key select the installer drive and let it run!

OK, so thats how to do it but why??

What is your reason? You may have another issue here! If you have a performance issue I would suspect the HD SATA cable is worn and/or the HD needs some TLC. Before you make the jump how about giving us a bit more here. We might surprise you on fixing things without going so extreme.