This is an informal expression used when asking, or instructing, someone to be quiet or to calm down.

This is a humorous term to point out that a person is to excited. They are being hyper like a child - and they may or may not be a child. It can also be used for any extreme emotion including happiness or anger.

We would say "turn it down a notch". Lower the thermostate a notch, or lower it one or two degrees.

calm down or lower something, like your voice or if you're very bouncy on that particular day.

dial here means turn. dial it down a notch= turn it down a notch(a step, a degree)