All you have to do is hit "U" on the upper right hand corner. It's also very important that "P" is on for persistence - this continues to track planes even after they go black. I've had the exchange open since the early morning so I'm tracking a bunch of planes that have already gone black. If I open a new ADS-B exchange tab then all I see are 269 planes because the ones I'm still tracking on my old window don't show up since "P" wasn't on in that tab.