There are plastic tabs that push through holes in the metal under the bumper cover. You have to be really careful not to break the plastic tabs. If you get under the truck and remove the spare tire, you can look under the bumper and see the tabs sticking through. It's easier with two people. One person to push the tab over while the other person can pry the plastic cover up very gingerly. Don't try to pry the cover up unless the tabs are pushed over. Start at one end and work your way over.
Hope this helps.