"modify" like "edit" or "fix" something .. "change" something by other thing .. ex: you modify the color of your shirt from blue to green .. means "you colored it green"(the same shirt) but if you said you changed it .. it's mean you exchanged it with green one(you get other shirt"

Modify to change some parts of something fix it or repair it or whatever change means entirely convert it to another thing not necessarily by exchanging it , maybe you just completely change it's shape/form

To modify is to add or subtract from what is that you are talking about. To change something can mean to make it something entirely different.