I have seen these with a single countersunk screw attaching the rotor to the hub. Just look at the outside of the rotor and you will see the holes for the wheel studs to go through. Look closer and you will see a screw head (it could be a slot, an allen, or a torx) and it will likely be very rusted over. I have also seen these screwed in from the back.

This screw needs to be removed and the hub and rotor will come apart. All the screw does is make it easier to reassemble the thing when you are replacing it on the car. It needs to be there, but it is not like it is a vital part of the structure so it might not stand out and you will need to look hard for it.

But if you cannot see how to do this due to the part being rusted or perhaps just different from how we have described it you can always take it to a brake shop and for a small fee they will deal with this.

It might cost you $10, but what is your time worth?