Phone posting; I'm still a beginner and my methods tend to get funky on more detailed images, there's probably better ways but what works for me:
Download Gimp (quality may not be as good but I'm a cheap bastard and free photoshop)
Take the picture you want to edit, use paths tool to outline the skin (you can change the curve on a line between points two by dragging it, make sure to zoom in and close by overlapping final points, where you'll spend most of your time to get it just right)
once done shift+v to select from path, then right click>paste as>new layer in place, go to layers window set mode of pasted layer to multiply (makes the finished result look more natural),
open up the 'color wheel' image eyedropper the tone you want to change to along with the original tone (on the multiplied layer) use the pencil to add in a corner and undo them (helps for next part), from the top colors>map>color exchange set the colors to change and set each RGB to max, OK and done
go to file>export to save as .jpeg ('save as' creates a bloated gimp file)
Tldr practice your path selections and remember to set your layers to multiply before color swapping.