Loosen the screw holding the rear upper track support of the bifold door. You'll find this on the side of the door closest to the door jamb. Once the screw is loosened, you can slide the support back and forth along the track.
Place a level onto the narrow vertical side of the door. This is the side opposite the upper support track from Step 1. Slide the upper portion of the door back and forth. Center the bubble on the level. When the vertical side of the door is plumb, the door is level.
Tighten the screw on the rear upper track support until it doesn't move. The door is now level and should swing freely without binding in the track or rubbing the floor.