Just pushed on the rim underneath. Use a wooden or rubber hammer. Usually they shrink over the years and then they become loose. I repaired my one by cutting it at the bottom and using this method my japanese friend Take made
http://infinitier.ddo.jp/e32/page/mainte/mainte_051.htm
http://infinitier.ddo.jp/e32/page/mainte/mainte_052.htm