Well, the options you find when pressing the power button unfortunatly have changed with Android 6. You’ll have to get used to access them by swiping down the munu bar with two fingers. And to restart the phone, just power it off and then on.
Well, I doubt that rolling back would be a good idea in general since you obviously came from a pretty old software version which doesn’t include the latest security patches. Since you already own one of the few phones for which monthly updates are provided you should also install them. Last but not least this might help you once you should need to replace a part of your phone and the replacement requires new drivers, like e.g. the screen.