When your character dies, tap the Restore XP icon in the top right corner of the screen, 

or you can view the list of your XP and item losses through the Priestess of Nui in town.

You can use the free restoration 5 times per day, and once you've used up all the free restorations,

you can use Gold, Resurrection Scrolls, or Gems to restore them.

※ The amount of XP restored may vary, depending on the method of restoration.

※ If you die more than 10 times, your XP will be lost after 10 deaths, so be sure to restore it before that happens.