Prepare Your Mac
Even with the simplified upgrade systems in place on Apple computers these days, you need to prepare before you click install.
Use this guide to prepare your Mac for OS X El Capitan so that you are backed up, cleaned out and ready in case something goes wrong. Backing up takes some time, and may require a hard drive, but it is very important.
In most cases you won't run into a problem, but it does happen and if you don't have a backup you may lose photos, files and other important items.