The quickest and easiest way is to use something like Paint.net where there's a bunch of filters and tweaks for colour, contrast, hue, etc as well as some automatic levelling tools that can do it for you.
It's quicker than trying to learn how to use Photoshop.
If you can't work it out for yourself, just google "Photo software basics" or maybe look at a few YouTube vids to get going.
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