The software likely looks not only for words, but also for structure. You can change many words, if you are using the same sentence structure it will still be marked (correctly) as plagiarism. It isn't about words, it is about the way the ideas are organized too.
The real issue is that you need to learn how to do this type of assignment in a way that isn't plagiarism. I don't know what you are doing. My advice is to read the articles that were assigned, take some brief notes on the topic. Then to close the articles and write something original. The point is that you start with your own words, and don't look at the articles you are getting the information from.
If you honestly believe that you did your best on this assignment (you didn't just put this together last minute by cutting and pasting and changing words) then you should talk to the teacher or get extra help..
You should look at this as a learning experience. You have something to learn, don't go in looking for a fight.
Either go to the teacher for help on how you can do the assignment better, or find a tutor or extra help.
This is a skill you have to learn. It takes a little work, but learning to write well is pretty important.