Loosen the back allen bolt first and then you will be able to turn the front thumb screw. You will probably have to remove the rear bolt completely before you will be able to remove the seat. Try not to remove the front screw completely. If your seat has a cutout you can use an allen wrench for the front bolt also as there is a allen socket in the thumb screw.
To get the seat level if you do not have a cut out in your seat it takes a couple of attempts because you have to tighten the rear bolt last. If it is not level you need to loosen the rear bolt adjust the front and try again.