How do I uninstall Yosemite?

Everything I wrote about my difficulties with Yosemite above did happen. Then I decided to try simply rebooting my iMac (late 2009) twice in a row and there seemed to be a miracle. Yes, my computer with Yosemite installed started to work as good as it was before. I really can't explain it any further. GOOD LUCK!

Yosemite has slowed my computer unbearably and now Safari has to be force quit every time I use it; sometimes several times before I can use it.

Thanks for this post, I think it is putting me on the right path to solving my problems. This can be very frustrating and I appreciate your time and patience.