How do I go back to Yosemite

Did you ever find out how to switch back?

There should be at least 2 ways of doing this:

A:

1. Reboot and hold down Command+R to enter Recovery mode (You can also hold alt/option and choose “Recovery HD”)
2. At the OS X Utilities menu, choose “Restore From Time Machine Backup”
3. Connect the Time Machine drive that contains the most recent Yosemite backup to the Mac, then click “Continue”... etc.

B:

1. Re-download Yosemite from the Purchases tab of the App Store
2. Refer to the following support article for how to make an installer on a temporary partition/USB/external drive: https://support.apple.com/en-gb/HT201372
3. Reboot, holding alt/option, and choose the new installer partition/drive. It should be fairly straightforward from there.

Both of your suggestions do work. I had to do that on one of my MacBooks as well. I still like Clean Installs (#B) more, data might as well be transferred to another partition or hard drive prior to reinstalling.

hold down command + r

and click reinstall mac os x

follow the promts

YOU WILL NOT LOSSE ANY DATA