Why do all the religions, especially Jehovah Witnesses change up the bible?

It clearly says in Revelation 22:18 not to take away or add to...woe to them that do. Apparently the Jehovah Witnesses make a big deal of the word "a" being left out of John 1:1meaning that their is more than one god.....their is only one god, not more than one. And they don't believe in the trinity...which to me is practically the same thing as rejecting it, which is not a good thing. Can someone please explain to me?