How do you change battery in Kidde model KD cosm-1B?

The kidde.com website also has some directions. First twist the unit from the ceiling mount. Next you need to "squeeze" the white plug containing the red, white, and black wires in order to free the unit from the wall/ceiling. It looks like it's part of it, but it's a plug.

The batteries are in the round disc itself. Open the battery door (you can see a rectangular outline) by prying open under the tab near the edge. Remove the old 9V battery and remember to push down the red plastic flag before inserting the new battery. The unit will start to make crazy sounds for about a minute until it settles down.