If you've got a sloped ceiling, such as cathedral ceiling or loft ceiling, you'll need to decide how to deal with the issue of lighting. The ceiling electrical box (or boxes) for the overhead lights will be embedded in the sloped ceiling. You can hang lights at a slope, but they cast light oddly, hitting the adjacent walls more than the floor. Custom angled lights are available but expensive and hard to find. An easier solution is a standard angled bracket that will hook to the sloped box and create a horizontal plane for the light.
Cut the power to the old light fixture at the house circuit panel.
Take out the screws holding the old fixture to the ceiling. Lower the fixture enough to access the wires behind it. Disconnect the wires. Take the fixture completely off the ceiling, leaving the old wires dangling from the exposed electrical box. You should have a black (hot) insulated wire, a white (neutral) insulated wire, and a bare copper wire for grounding.
Set your angled cathedral ceiling bracket to the electrical box and secure it there with the screws it came with. Hold your level along the bottom of the bracket and adjust the angle of the bracket until it's level. The exposed wires from the electrical box should still be accessible through the bracket.
Hook the copper ground wire from the box around the green ground screw on the angled bracket. Tighten the screw.
Hold your new light fixture near the bracket. Connect the white wire from the light to the white wire from the box by twisting a wire cap over both wire ends. Connect the black wire from the light to the black wire from the box in the same manner.
Lift the light up to the bracket, pushing the two wire connections into the electrical box. Connect the light fixture to the bracket with the provided screws.
Turn the circuit back on and test the light.