The public beta of macOS Sierra is now available, so anyone who wants it can install the latest operating system from Apple. In this case, you just have to register in the public beta program to download it.
If you have already done this and you have tried the system for a while but it does not convince you, do not worry, you can easily return to OS X El Captian. This is what you should do.
Make a backup
Obviously, the first thing we have to do before performing any system installation is a backup. You can either use Time Machine or manually copy the documents you are interested into an external drive. Once done you can continue with the tutorial.
How to erase macOS Sierra
- Make sure your Mac is connected to a Wi-Fi network
- Press the apple icon on the menu bar
- Select the Restart option
- When you hear the start sound press and hold Command + R
- Choose Disk Utility
- Click Continue
- Choose Boot Disk
- Choose Clear at the top
- Choose the main disk (or where you have MacOS Sierra installed)
- Selects Mac OS Extended as the format
- Click on Delete
How to Reinstall OS X El Capitan
- Start Disk Utility
- Click Reinstall OS X
- Click on Continue as soon as you get the OS X window El Capitan
- Follow the instructions that appear
The system will download again and then restart the computer to complete the installation. After that, you can restore the copy of Time Machine or copy the saved documents.