Video is simply many still images that are played at a fast speed to look like motion. Editing a face in a video is based on this principle as each frame (image) will be edited. Since most films play at 24fps (Frames Per Second) there are a lot of images to edit. Fortunately with the use of special effects software this process is somewhat automated through a process called "Key Frames" that remember settings from previous frames. Still, the process can be quite tedious. This can not be done using native Windows software. Programs like Adobe After Effects, Foundry's Nuke, Autodesk's Combustion and Apple's Motion all are capable of editing a face in video. These software packages range in price.
Download and install a special video effects software like Adobe After Effects, Foundry's Nuke, Autodesk's Combustion or Apple's Motion. Learn the tool bars and menu controls, as each program is different. All of the aforementioned software suites come with helpful manuals to help orient new users.
Decide on the type of edits you will perform. Knowing what the final outcome should look like before beginning to work will work as a road map for the editing. There are countless possibilities for editing a face. Some of them include adding makeup, changing bone structure, colors, features, tints, hair and eye color, expression and skin. There are also morphic features that change the face gradually over time. This can work well for turning a person into a zombie or a man into a woman.
Use frame-by-frame editing. For the most accurate results, frame-by-frame editing is best. This process is painstakingly slow, but it is a popular method used by professional digital animators and digital effects artists. Use the paint brush tools to manipulate and draw on the video frame (images.) Go frame by frame and edit or draw the changes.
Use key-frame editing. Key-frame editing automates the process. Start editing the face at the start of the video or first frame. Set "key frames" for all the effects. This will tell the program to remember what settings were used. Forward the video and make edits as needed; the key frames will automatically adjust and the settings will change as the video plays. This saves the tedious process of editing frame by frame. In a video with lots of motion, this process may require more work.