Removing a headlight assembly is a fairly simple job, but the only tricky part is keeping track of the mounting screws and the proper alignment of the electrical connectors. The vehicle's owner's manual should show the exact removal process specific to the make and model.
After finding the location of the mounting screws and electrical connection attachment point for the headlight housing, the next step typically involves opening the hood and taking the fasteners out. Having a tray or box handy to hold the screws keeps them from falling onto the floor of the garage. The electrical wiring from the car to the headlight is disconnected at this time as well.
After removing the light and housing, the light is pulled out. If the assembly has a significant amount of damage, this is a good time to replace it with a new one. If there's no damage, the new light should be installed in the old housing. To complete the job, the assembly and light are slid back into place, the electrical connectors are hooked up again and the fastening screws go back into place. For safe nighttime driving, the headlights should be aligned according to the instructions in the owner's manual.