There are tabs on the bumper that snap into a plastic clip that runs along the area where the fender and bumper meet. As long as nothing is broken, it should just snap back into place.
If you zoom in closely on your picture you can see a couple of the tabs.
Prior to beginning, try to see if the tabs are pointing straight in. If they are bent, you may need to try and straighten them some. You just need to make sure that they will all go into the groove of the plastic clip. There is probably a screw on the inner wheel well that needs to be backed out as well which goes through the bottom tab.
Push on the front corner of the bumper while simultaneously pulling up on the wheel well portion of the bumper and get the bumper tabs started into the clip. Once it is lined up, give it a few good smacks with a closed fist and it should lock into place. Having a helper to push on the bumper might make the job easier.
If the tab that the screw goes through is broken, you may have some extra work to do.