I have practiced this a lot personally and I’m continually changing my focusing points when shooting portraits. I can say it’s one of the things that improved my image sharpness dramatically.
In this video Mark covers the focusing points on both Canon and Nikon cameras. he is using some older cameras but this info is still very valid. If you own a different camera you’ll need to refer to your manual but this will teach you the underlying principles necessary. For demonstration purposes he uses a Canon 5D Mark II, the Canon 7D and, the Nikon D7000