Cut power to the existing fixture at the fuse box. Climb your ladder to access the light. Take out the screws holding it to the ceiling. Lower it enough to expose the wire connections behind it.
Disconnect the wires by unscrewing the wire caps holding them together and pull the light away completely. You should be left with three wires hanging out of the ceiling --- one black wire, one white wire and one bare copper grounding wire.
Find the mounting bracket for the new light fixture. Hold it to the electrical box in the ceiling and attach it there using the provided screws. The wires should still be hanging free, around the bracket.
Wrap the bar grounding wire from the box around the green screw on the bracket. Tighten the screw.
Carry the new fixture up the ladder. Connect the wires from the fixture to the like-colored wires from the ceiling box (white to white and black to black). Connect them by holding the wires side-by-side and twisting a wire cap over both of them at once.
Hold the fixture up to the box. Connect it to the bracket with the screws that came with it. Look at the light from the floor to make sure it doesn't have to be twisted in one direction or the other, so it appears square in relation to the walls of the room. Restore the power.