I'm afraid there's not really a quick and easy answer to this. If the person is pretty static in the video, it won't be much of a deal. You can just place some overlay picture from the program or your own image file over the video and use the size/position settings to resize it to just cover the face. Manually adjust the position somewhere along the line, if necessary. If the person is actually moving, you would have to explore the program magnetic tracking feature. This will still mean a lot of manual adjusting and might be a little tedious. Maybe this  YouTube tutorial here will help: