If its the bmw system top box, you lift the handle whilst depressing the round npush button just above it, this will reveal a flat metal plate that twiats, twist it as far as it will go, this will then allow the square plate at the bottom of the top box below the handle to be pulled out, pull this out and your box should come off.
Hope this helps
Pull the handle up and twist the bar that you'll find hiding underneath it (can't remember which way to twist). That'll release the metal 'clip' that holds it to the rack. At least, that's what I remember.....
As Steptoe said, but if it's getting the box from the mount I'd leave it on as mine fell off after cleaning it up, apparently they have a tendancy to do this now and then, now it's well held on with cable ties too!