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Screen shot wouldn't help.  I didn't know you have PS to use, so I'll give you all steps.

The procedure is to 1) duplicate the layer, 2) on the new layer> Image>adjustments>Replace color, 3)  click the eyedropper in the grey area, 4) in the pop-up, move the Lightness slider to the far left.  This darkens the grey to black.  Paint over that upper left corner with black.

Since your background is grey, and there's black and grey in the dog's coat, this will add unwanted black to the dog's coat.  To remove that,

make that layer a Layer Mask and use the brush set to black (remove) to erase the effect on the dog.  The lighter coat will then be visible.  A little care in the edges is required with a small very soft brush.  You'll get a little halo effect that can be corrected in the Layer Mask by going over the edges.

A tip for future photographs...When you know the end result will be a cut-out like this, use a background that is easy to remove.   My kit includes a couple of yards of bright green cloth and a couple of yards of sky blue cloth  purchased from a fabric shop.  Green would have been good here as a background since there's no green in the dog.  No Layer Mask treatment would have been required.  It's similar to using a Chroma Green process, but cheaper than buying the program and a dedicated Chroma Green background.    Place the background well back from the subject and use a wide open lens so the detail of the fabric isn't picked up. 

Try the above and let us know how it works for you.