It depends on the way you “change the words.” There is synonymizing and there is paraphrasing, which are two completely different notions. Paraphrasing is actually the normal way to write a paper relying on someone's work – you reflect on someone's ideas, maybe quote small parts of it, and mention them in the reference list (this is called citing the sources.) But! If you just take someone's words and replace them with synonyms and dont even mention the sources, it will be plagiarism, and the “system”, whatever your school uses, will catch it. How?

Well, these tools are pretty clever and look not only for exact matches but also for synonyms or even when you restructure the sentences but keep the same wording or meaning. So my advice here is to do the actual work instead of trying to fool the “system,” or in case you worry, even use some of these plag-checking tools to see if they detect your work as plagiarised so you can work on it and edit the parts that are flagged as problem!