there are two tabs on the left and right sides that have screws in them. unscrew them. there is a screw in the middle. unscrew it. at the bottom, the grill is clipped to the bumper. give it a little tug and it will come off. slide the grill out. most aftermarket grills will slide right in, some may not have the clips at the bottom.
the plastic screw rivets
take a phillips head screw driver and turn them 90 degrees counterclockwise while you use your other hand to push up from the bottom. they will pop up. (don't use a lot of downward force on the screw driver or the 'screw' head will get mashed)