How to remove light fixture
One of the three clips will be spring loaded.
Pull straight out on it, while supporting the glass, and then the glass will be able to swing down and out of the other two.
Once the glass is removed you can access the fasteners, two screws or nuts, that are holding the fixture to the box in the ceiling.
Turn the breaker off that is supplying power to the light before you do anything with the wiring.