To burn your Burner number, scroll to the number you want to burn then tap the flame icon at the bottom of the page. If you're on "All recent" when you tap the button, you can pick which number you want to burn.
To go through with the burning, press and hold on the burn button till the Burner logo fills up all the way. You will then be presented with a confirmation message asking you if you're 100% sure you want to burn the number. ATTENTION: YOU CAN NOT REVERSE THIS. ONCE A NUMBER IS BURNED, IT IS GONE AND IT'S DATA IS DELETED. DO NOT GO THROUGH WITH THIS UNLESS YOU ARE 100% CERTAIN YOU NO LONGER WANT THIS NUMBER.