Order replacement insoles, which are available from the Ugg website. Not every Ugg boot design has replaceable insoles, so double check your boots and choose the proper insole and size you need.
Reach inside your Ugg boots and carefully lift the current insole away from the heel of the boot and remove it.
If you're replacing the insoles because your Uggs got wet or sweaty, air them out for a few hours. Sprinkle in a little baking soda and let sit overnight to help remove odors.
Remove the new insoles from their package and insert, toe first, into the boot. Push the toe of the insole into the boot as far as it goes. You may need to wiggle it around a bit to make sure it's all the way in. Once the insole is in as far as possible, push the heel of the insole down with your fingertips.
Once the new insoles are in place, put the boots on to mold them to the shape of your foot. If there's any bunching, remove the insole and repeat Step 4.