Re-capping a bottle is easy. Lava brand lava lamps need a bench capper to be capped properly because a hand crimper will not work. To re-use the original bottle cap use a flat head screw driver to gently pry the cap off by working your way around the cap.
If the cap is a solid-crimp cap you will need to be careful while taking it off to not damage it or you will not be able to put it back on. Standard bottle caps fit some solid crimp caps.