If you look inside the trunk, there will be two large orange rubber plugs in the trunk pan. If you remove these plugs, you will have access to the upper shock bolts. The shocks just use a regular bolt/nut. You will have to hold one end while turning the other. Probably be easiest with a helper. One to hold a wrench on the bolt and another to run the ratchet inside the trunk.
Don't be surprised if the back end sags really low once the air shocks are removed. These cars sat low from the factory even with brand new springs. You can get new springs OR you can disassemble your spring packs and install and extra leaf in them. If you aren't that experienced with working on cars, it would probably be easier if you just replaced the springs.
Also, air shocks are known for damaging the upper shock x-member. I used to have a 73 Challenger with air shocks. The x-member was completely separated from the frame rails due to the extra load from the air shocks. Make sure you have a look at yours and have it repaired if needed.