There's a lot you can do. I'd recommend checking out the subreddits SkincareAddiction and SkincareAddictionUK - they have a wealth of knowledge.
At the very least though, you should be wearing, SPF30 minimum (and make sure it's photostable and broadspectrum!). This will help prevent the hyperpigmentation getting any darker.
Also, it might be PIE - or Post-inflammatory Erythema. The easiest way to tell the difference is to press on the areas and if it blanches (goes white) it'll more likely be PIE than PIH (also, PIE is more common in those with fairer skin, but can happen to people with deeper skin tones, and vice versa). For the most part though, the methods of treatment for PIH are largely the same as PIE, but it'll take longer to go away than PIH will.