Open your picture :
First, decide from which part of the image you want to remove darker shades. For example, I have targeted the area drawn by the black lines :
Now, go to Select > Color Range. Click on the areas in the image where you want to remove dark shades. Use the middle eye dropper tool (add to sample tool) to add the area to your color range. If you accidentally added more areas, use the third eye dropper tool (subtract from the sample) and click on the areas where you want to subtract the color range.
When you click ok, the selection will appear. Press control+j to duplicate the layer and change the blending mode to screen.
You will notice in the previous image that there were many areas which were not in my targeted area. Now, use high feather eraser tool to erase the extra areas. I have used 65 pixel eraser tool to erase those areas which I don't want to lighten. I got something like shown in the following figure. I guess, I am well in target :)
Finally, go to Filter > Blur > Gaussian blur and adjust the radius until you get desired result.
Compare the before and after images and you will notice the difference :)
Before :
After :
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