Are you allowed to copy something from the internet change a few of the words and say that it is yours?

Change a few Words? No. You have to do much more than change a couple of Words. If the essence is the same, and it is recognizable as such, it is a copyright infringement.

And you should know that teachers and publishers have the ability to search the web for entire pages of material and compare them for such violations. Just changing a couple of Words is not going to prevent them matching it up.