This effect uses the versatile Hue/Saturation adjustment. Load the adjustment layer (Layer>New Adjustment Layer), so, if you need to, you can go back and edit the image at a later stage.
The next thing to do is to pick out hand symbol that has the two arrows on. This is a tool that lets you click on a colour directly over your image, rather than having to make a selection beforehand.
With the hand symbol selected it’s just a case of clicking on a part of your image that you want desaturated and dragging your mouse to the left – it’s that easy!