Question: gow how to upgrade axe?
Answer: Once you've got the Chilling Mist of Niflheim, go to one of Brok or Sindri's shops and trade it for a Frozen Flame. You'll find the option to trade in the Buy tab under Resources. Once you've got your hands on the Frozen Flame, you can fully upgrade Leviathan Axe and grab the 'Worthy' Trophy.