Fix a dent in a plastic bumper with the application of heat, either with boiling water or a hair dryer. Heat makes the plastic softer, which in turn makes it easy to pop out the dent, even with your bare hands.
To fix the dented plastic using boiling water, boil a big pot of water, and pour it over the dented area. Be sure to pour the water slowly to give the plastic time to heat up. After pouring all the water over the dented area, reach under the wheel well with your hands, and push the dent out. Once that's done, leave the bumper to cool down and harden.
To fix the issue with a hair dryer, aim the heat from the dryer at the dent to warm the bumper up. This may take anywhere between 10 and 30 minutes depending on the thickness of the plastic, so make sure to use a heavy-duty dryer for the task. Once the plastic expands and becomes flexible, place your hands behind the dent, and push the dent out.
For optimal results, use two hair dryers, aiming one at the dent to warm the bumper up and then aiming a second one at the spot simultaneously for 10 minutes or so. Follow up by using the single hair dryer for an additional 20 minutes more before pushing the dent out.