You can't without breaking warranty. And even if you did, you'll lose everything on the phone. What you do first is make a back-up of everything you want on your phone, put a copy on a PC or something, then factory reset the device. This won't bring back kitkat, but it would usually make the phone work better. Major upgrades are sometimes difficult, that the best way is often to do a reset before and after an update.
Normally the phone should work better after this. If you still want to bring back KitKat, You'll have to go to the rooting forum for your device.
And yes there is a silent mode feature. It's just set differently this time. It would be under Interruptions Mode. However, since this new Google feature has had some backlash, the latest update for the S5 brought back the old Silent Mode. A more recent update for your Note would likely bring it back too.
