Bumpers are usually held on with a series of plastic push pins. It's meant to be an easy R&R. Lay down under the back of the Flex and poke around with a flashlight, your will see what i mean. Wear safety glasses, lots of grit will start falling down in your face.
The bumper guard is usually a piece of styrofoam or plastic eggcrate. If it isn't too compressed, re-use it.
The pins look like these.. pull out the center pin and then pull the entire plug out of the hole.
http://www.cjponyparts.com/ford-fron...-2004/p/HW834/