Assuming you need to replace the entire headlight assembly:
To remove the old headlight assembly, first disconnect the negative battery terminal to prevent any electrical shorts. Next, use a screwdriver or a trim panel tool to pry off the plastic rivets that hold the front bumper in place. With the bumper removed, you should now have access to the back of the headlight assembly. There are three bolts holding it in place; two at the top and one at the bottom. Remove these bolts and pull out the old headlight assembly.
Installing a new headlight assembly is simply reversing this process. Start by putting the new assembly in place and then bolt it down. Reconnect your car's battery and test out your new headlights!