How can I correct shadow/highlight with free software?
If you have Davinci resolve, use it. If you do have it, get it and use it :o)

There are several ways to handle light and shadows, you just need to understand that lights are normally the light zones and shadows, the dark... Pretty obvious.


On Resolve, you can use the color panel.





Boost or lower the highlights on the gain wheel.
Or use the curves and
Move the curve.
And see the results here.


There are more advanced ways to do it, like selecting color ranges, and you can find tutorials on the internet. But the tools are there.



Keep in mind that on the overexposed areas, if the range is clipped, no program will recover information where there is none to recover.