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It is not very easy to downgrade to Yosemite or lower version from El Capitan.
For that you have to download a copy of Yosemite or previous version of OS X from App Store, then create a USB bootable drive using a third party application named Disk Maker and install from that drive. You can only download Yosemite or previous version from App Store if you have downloaded them before.
After creating a bootable USB drive, boot your Mac to that USB drive and then you have to erase the hard drive to install Yosemite or previous version. This process will delete everything from the hard drive and install a fresh OS X. So you have to backup all your important files to an external hard drive before you do this.
Follow the steps below to create a Yosemite or lower version OS X drive.
1 - Backup all your important files to your hard drive. Open App Store application from Applications folder
2 - Click on Purchased tab and if you see Yosemite or previous version, then download it.
3 - Then download this application Disk Maker to create a bootable USB drive --> http://diskmakerx.com/
4 - After creating the USB drive, connect it to your Mac.
5 - Restart your Mac and immediately hold down Option key on your keyboard and this will bring Startup Manager
6 - From Startup Manager, select the USB drive and press return and this will boot into OS X recovery screen.
7 - Select Disk Utility. In Disk Utility, select you hard disk in left column and then click Erase under Erase tab. This will erase the hard disk.
8 - Then close Disk Utility and click Install Mac OS X and follow the onscreen instructions to install that version.
If you don't see Yosemite or lower version of OS X under App Store, the above steps will not work and you have to make a Yosemite or lower version drive from another Mac running Yosemite or lower version using the above steps.
If you have any questions or would like any additional information, please don't hesitate to ask.