Hello.Thanks for your query.
For any hyper pigmentation , you should be using a broad spectrum sunscreen and repeating it throughout the day.For an acne prone skin , a gel based sunscreen is preferred.This will prevent the patches/spots from getting darker.
To lighten spots/patches,you can use Vitamin C serums, Creams containing Kojic acid
,Face wash having Glycolic acid
You can visit your dermatologist
who can prescribe you chemical peels
to lighten the skin in a better and faster way.
Hope this is helpful.Take care!