That's easy. You just need to get rid of the panels on the left and right side of your trunk. Therefore put both your rear-seats in a downward position first. Afterwards lift the panel on the bottom of the trunk. You will see your battery, combox etc. If you have the "storage compartment package" (sorry, don't know If that is the right word for it) you have to loosen two Torx screws now (see photo - 1.). After that find 4 knobs securing the bumper protection and take it out (photo - 2.). Now you can get rid of the left and right panel by just taking it out. They are not secured with screws etc. Be careful on the right side though as there is the LED for your trunk and in some cases a power plug.
Now you can take out the speakers. They are secured with 3 screws each. If you want to upgrade your speakers with tweeters you need to change the speaker panels now.
