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.

Again, there can be technical differences, certainly in engineering design and standards they can mean slightly different things, but in common usage they are the same. Revise tends to be more paperwork oriented and of course when you prepare for an exam you "revise".