How do you use stored EXP?

Stored EXP is a catch-up mechanic that lets you gain bonus exp when leveling up. If there are days where you haven’t logged in, you’ll get Stored EXP. This will appear as a purple color in your EXP bar where you'll get additional bonus experience.

In order to use it, simply do activities that provide you with regular EXP. Stored EXP will be added on top of that until you finish up all stored EXP you have.