The trim ring clips into place on the ceiling assembly, so you will need to rotate the trim ring counter-clockwise until the clips line up, and then you will be able to gently pull the trim ring down. Under the trim ring, there are tabs that hold the diffuser and gasket in place. Twist the tabs out of the way to release the diffuser and gasket so you can have access to the inside of the tube and the light kit if you have one.
When putting the trim ring back in place, make sure the gasket around the diffuser is not sticking out in places (if it is, just push it back into place), then line up the clips, and twist the trim ring clockwise until it is snug.
Here is a short video that demonstrates removing the trim ring and gasket: