How do you change the parking lights on a 2007 Subaru Forester. The owners manual doesn't explain.

Simple, but not as easy as it sounds. It is a twist-off T10 fitting, but quite awkward to get to. You need to remove the six plastic hold-downs across the top of the coloured moulding which forms the front of the car, just in front of the radiator. This is easy. You simply hold down the outside part of the "hold-down", while pulling the centre post out. Then you can simply pull out the remaining part. Don.t lose them! Then you need to carefully pull the top of the moulding towards the front of the car, (careful you don't break it!) and place something (bit of wood, maybe) between it and the bar across the front of the radiator, to give yourself (a little) room to work.
Then, unless you have very slender / nimble fingers, you will need long-nose pliers to twist the light fitting base anticlockwise until it stops (about 1/4 turn), before pulling it out. Then, it's simple to pull out the T10 bulb and replace it, and reverse the above procedure to reinstall.
I have only just finished doing this five minutes ago, replacing the bulbs with white LEDs.

This is general knowledge. It either has screws on the outside of lamp to pull it off (not likely since this is an 07) Most likely on the inside is a way to twist off the bulb holder, usually a quarter turn is all it takes. Then you can remove bulb and find exact replacement at autoparts store. Sometimes you can get free help at the big chain autoparts store. Good luck!