Q: How yo revert from OS Captain to OS X Yosemite?

Your options are limited.

OS X El Capitan: Revert to a previous OS X version

1.- I tried to re-install OS X Yosemite, but it's not available anymore on the App Store,

It should still be avaialble under the "Purchases" tab.

You'll need to boot from an external source to be able to erase the internal drive and install Yosemite.

It would be a better idea to fix what's wrong with your Mac rather than attempt to downgrade to an earlier version of OS X, which you now know might not be an option anyway.

Back up your Mac if you have not done so already. To learn how to do that read https://www.apple.com/support/backup/. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac.

If your Mac is momentarily unresponsive, consider the following.

Launch the Console app - it is in your Utilities folder. You can find it by selecting Utilities from the Finder's Go menu.

If the log list column on the left is not already displayed, show the log list by selecting Show Log List from Console's View menu. Select Show Toolbar if it is not already shown.

Locate system.log in the list and select it. Many date and time-stamped entries will appear, hundreds of them, and you must find the entries relevant to your Mac's problem.

To do that click the Clear Display button in the Toolbar. All previously displayed log entries will be disappear.

Next: Perform whatever actions cause the Mac to exhibit the slow behavior. If the problem is caused by errors logged by the system, the Console window will show them being recorded in system.log.

One or more of them, along with their time stamps, may reveal the reason for the problem you describe.

Copy and paste those log entries in a reply. If hundreds of the same repetitive messages appear, please edit them before posting. There should be no need for more than a few log entries.

Most of the entries will be cryptic but will contain information you might consider personal such as your Mac's name. If you do not want that information to appear, delete or obscure it when posting your reply. Leave enough information so that the entries can be deciphered.

Let's try to fix the problems you're having with El Capitan before doing anything drastic like downgrading your OS.  Go to http://www.etresoft.com/etrecheck — download and run EtreCheck, copying the status report to paste here.  You say that El Capitan is very slow and laggy; that could be the result of third-party applications/extensions/plug-ins on your system bogging it down.  What EtreCheck does is give us a glimpse of your system so we'll know what's on it.