It's as simple as selecting the area of the image where the face is and imbedding it into the other photograph.
The only thing you have to do, once you have swapped faces, is adapt the cutout to the same proportions of the image: size, saturation, brightness, contrast, pixelation, etc.
That's all there is to it: open a file, select the area where the face is, and place it over the other image.
Use the Zoom feature for help to make editing the photograph easier. An advantage to this version is that the trial version doesn't insert a watermark on the images, so you can use it without any hassle.
1-day trial version.