Take out the two screws (Phillips) and Undo the silver clip with a screwdriver. Slide the speaker box out (it would be best to do this with the rear seats folded down) and unplug the wiring harness.
In the picture, the screws are circled and the clips have arrows. The silver one is on the right.
In the trunk, you need to pull back the carpet to see the USM. Here is a very painstakingly drawn representation of where the speaker is in relation to the USM.
Once you pull back the carpet, this is what you will see (this is on the oppostie side of the car). The dust cap should be pulled off to see the top bolt. Do not unbolt this now. Only unbolt the side nuts. (13mm, btw).
This is what it looks like with no dust cap: