Open both images. Go to the image with the head you wish to keep. Duplicate the layer (Ctrl J) & use the “quick selection” tool to select the head. Edit>Copy.
Now open the image with the body in it & Edit>Paste. Click on the “move” tool to position the head. Ctrl T (transform image) will let you adjust the size of the head until it looks similar. You may want to decrease the opacity of the head layer in order to match the face below as accurately as possible. Click the tick to confirm once you’re satisfied.
Ensuring the head layer is highlighted you could use the “eraser” tool to rub out any untidy bits around the head eg. the neckline. However, a less destructive way to achieve the same result is to add a layer mask Layer>Layer Mask, ensure the foreground colour is black & use the brush tool to erase any bits (this way if you erase too much you can turn the foreground colour to white & paint them back in). Next, to help blend the two images together alter the opacity of the eraser to about 12% & using a soft brush, go around the entire head to feather the edges.
If you need to match the skin tone a little better, again with the head layer highlighted, go to “hue/saturation” & alter these settings until you get the desired effect. You could also use the “curves” adjustment after this step if you have the full PS software.