If there's a black cover that you can see the speaker through, then I am assuming that you have the LED array for your lights. You'll have to go in through the back seat of the car and remove the plastic cover that sits partially behind the back seat and underneath the rear window.
Inside of this cover are speakers and your high mount light assembly. The light itself should be easy to remove at this point once you have the plastic cover piece out of the vehicle. I believe just a few clips and screws hold it in. Then, you just swap out the whole light unit since it's the LED version and replace everything the way you took it out.
Here's a youtube video that shows you how to do the light change. https://www.youtube.com/watch?v=pRXdqLtCAMs Good luck!
Edit: for the sake of being thorough, thought I'd add how to change the bulb if it's not an LED array. The single bulb version is very easy to change. You pop the trunk and look right underneath where the light assembly sits and you'll see a wire leading to the bulb socket. The socket just twists to release the pins and then you can pull it out. The bulb itself should be gently pulled from the socket and then the new bulb can be pushed in. BE AWARE: it's a good idea to use a clean cloth or even a glove to push the new bulb in because oils from skin can shorten the life of bulbs.