The catch here is that you're looking for something that is free.... The very quick solution that I would use in your situation is open the file in Adobe Photoshop and use the 'Auto Tone' correction. This nearly always produces a better and more natural looking image. I think that many other software packages will have something similar, but as I'm a happy Photoshop user, I'm not that familiar with the free alternatives.
If you're not that happy tinkering with computers and software, I would suggest going for something with some good quality automatic tools. Adobe run a subscription based service these days, you could consider paying for a single month or a short period so you can process your photos, and then cancel the subscription again. I don't know but perhaps they have a trial version that you could play with first so you know what you're in for.
I suspect it's something like this that you're looking for:
This uses Lightroom but there are similar tutorials for Photoshop.