I died it too dark. Can I just buy a light hair die and like 20% developer and lighten it?

Hm. You could try but its really hard to lighten when you've died it darker. Do you have blonde hair?
Did you die it recently? My step mom is a hairdresser and she told me about washing hair with dish washing soap so the die doesn't set in permanently. Kind of like the opposite of what those color protecting shampoos do. I say you try a more natural method so you don't damage your hair. Try henna, or some natural lemon citrusy based solution. If you just died it today, I would wait it out for a couple days before trying any of these. If you're going to dye it again or do henna, wait for about a week or two before doing so. Good luck! I'm not much of an expert. Just from what my step momma has said and personal research.

I only need to go a level lighter to match my extensions.

And my hair was level 5 brown and I died it to a level 2 brown so its pretty dark now

How long ago did you dye it? Like I said, its hard to lighten it up again, once you've died it a super dark color. It may fade. I bought a brown dye knowing it'd be black at first, and its starting to fade to a light reddish brown (I used to have red hair before that). Anyway, you might be better off dying the extensions that same color.

I wouldn't dye the extensions yet though. Your dark died hair might fade. Leaving it easier for you to brighten it with certain shampoos, like manzanilla or something