The words change and modify both verb words that are well used in the English language, as well as software languages. On a general note, change has to do with a process whereby there is a need to make a total replacement of something or somethings without the existence of the original or previous version.
Modify has to do with some sort of alterations or adjustments to something in such a way that there is a slight difference from its previous state. If care is not taken, you might actually take the word change to modify. Modify does not really mean change, but it can be said to be a "partial change."
The noun form of modifying is a modification, whereas modification is usually done to improve on an existing entity. Conversely, the word change can be used as both a verb and a noun in a sentence. Whereas, change means a total replacement of something; exchanging one thing for another.