and make it appear again when they go to the top of the page?
Do you mean you want the header to be just part of the page ie not fixed to the top or shrinking? there are two places to look and it depends on how its currenlyt setup. if you go into the header design options.
Under design, if Shrinking header is on turn that off.
The other place is under the content tab (same location) where sticky header can be set.
If both of these are off the menu will act like it's just at the top of the page. Scrolling out of view as you scroll down and coming back into view when you scroll back up to the top.
That would require some Custom Code. I believe you are trying to achieve something along the lines of
When the user scrolls down, the header moves up and is hidden. Then when the user scrolls up, the header scrolls down and is visible.
Am I correct in my assumption?