first open your images in photoshop

you need the image which is missing the person(lets call it MAIN), and an image of the missing person(lets call it IMG2)

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in MAIN, go to View>actual pixels to see it larger

then open IMG 2, using the crop tool, cut out the missing person

(if you have a more advance understanding of PS use the marquee tool)

now using the drag tool (arrow with the x) click and drag the picture onto the main image

drag it to where you wish the new person to be, and use the eraser (using a smooth and sharp eraser edge) tool to shave away any parts of the other picture

when done go to image>layer>flatten layer

and save