At the back of the platform, above the nameplate there is a thumbscrew. This thumbscrew is the trigger for the Z endstop. By adjusting the thumbscrew you move the Z-zero point up or down. Screw it down to move the nozzle closer to the platform for the first layer, up to move it further away!
Rev B machines also have the endstop mounted on bolts in slots, so you can move the endstop to make large adjustments to the zero point.