You don't have to! Uuni will burn pellets down to very little ash which can be disposed of when completely cool. If you would prefer, you can remove the Grate using the Hopper Lid and extinguish any remaining embers in a large metal bucket of water. This will help your Uuni to cool down a lot quicker. Please do remember to wear heat-proof gloves when doing so.
We have our Uuni Burner Quick Start Guide which shows you how to do this.