I saw on another thread that you might want to use a sony or panasonic battery, other people say that there are problems with other brands.
I used 1/8 flat screwdriver between the compass and the rubber shell. This is at the lubber line opposite of the tab (where you take the reading when diving). The bottom compass tab is almost exactly 1" below the surface, so you can use this to verify how deep to jam the screwdriver. Make sure that you are further than 1"... Gently pry it out using the tab for leverage, and yes hot water does help, thx Mark!
Once the compass pops out, then the computer pops up through the top. rotate the ring cw by pressing in center while turning with screwdriver against tab.
Hope this helps, I'm having trouble getting the ring out. Make sure that no water/moisture gets in battery compartment and replace o-ring and silicone grease.