Sprinkle a generous amount of baking soda at the bottom of the pan and let it sit. After an hour or so, give it a good scrub, but don’t rinse the baking soda. Fill the pot with water to cover the burned area, put it on the stove, and bring it to a boil. Let it cool, scrub again, and wash as usual.
You can also try vinegar, dish soap, or a plastic scouring pad. Not all at once.