I'm not sure why you have to ask such a thing, just look them up, but well, since I bothered reading this, might as well answer it I suppose. The word "change" is used when you mean something or somebody is different from what or how they were. (same thing or same person) E.g: You've changed (You are no longer the old you) The word "replace" means that you use somebody or something instead of somebody or something. (another thing or another person). E.g: I'm gonna replace my PC (I'm gonna get rid of my old one, then I'll buy a new one)

Change and replace can be interchanged because they both mean the same thing. "Replace" does sound a bit more polite and formal than "change" though. "Change" is a bit more general and can be heard more frequently in real world speech because it is one of the words we learned in school early on while "replace" is something we tend to learn later on. You can see "replace" quite a lot in books or hear it in polite speech. However, both can be heard in real world speech and using "change" instead of "replace" and vice versa is fine. Hope I've helped you. (^_^)

Also Change smth usually means: To change smth to smth new And replace: Change smth to the same thing Also You can change smth in your life, like the way of doing smth But to replace it - means only to do smth instead, for example)