Depending on the year Subaru Forester you have, changing the headlight bulb should not require removing the engine or the front bumper. The battery might be slightly in the way for the driver's side headlight, but you can just remove the hold-down device for the battery and tilt it backward to gain access.
Open the bonnet to the Subaru Forester. Turn on the headlights to note which one is burnt out and its position in the headlight assembly. This is also a good time to check any other bulbs that may be burnt out. Any item (like the battery) that restricts your hand from getting to the backside of the headlight assembly should be removed or loosened.
Locate the two wired plugs connected to the electrical blades of the halogen headlight. Remove the plugs by wiggling them off, and mark them to replace correctly onto the new bulb.
Locate the black rubber cover protecting the back of the headlight from moisture and other elements. Remove the rubber cover by squeezing it on the sides and pulling it off. It may give you some resistance, but it does come off.
Locate the wire spring clip that locks the halogen headlight into place. This spring must be pushed with a finger and lifted slightly to release it from the lock clip. It is hinged on the other side and will swing open to allow the removal of the bulb.
Remove the old bulb and insert the new one, making sure not to touch the glass bulb of the new headlight. Place the notch in the flange of the new headlight so it is in the top centre position when inserting it into the headlight assembly, and lock it in place with the wire clip. This will take some finger coordination, but be diligent.
Replace the black rubber cover in the same fashion it was removed. Plug the wires back into the blades of the new light bulb. Turn on the lights to make sure the new bulb is working properly.
Replace the battery hold or anything else you may have loosened or removed for access. Remove the tools and the old bulb and close the bonnet.