Modify tends to mean changing something but not enough to alter its identity. So you might modify a car, or a computer, or a plan, but it's still the same car, the same computer, the same plan, more or less. You might put bigger tires on a car, or more memory in a computer, etc.
Revise is indeed associated with editing written works, or ideas in general--someone might even revise their opinion about something.
To alter implies to change something in a way not intended by the original creator or in a way that obscures the truth or origin of something--such as altering a picture by "Photoshopping" it. Also, in tailoring, altering is to change a piece of clothing by making it smaller, or larger, or shorter, or longer, etc etc.