Ex wife filed papers saying I changed child support to a lower amount. I did not. How can I prove?

I'm not sure what you mean. Is she saying that you aren't paying all of the child support that's due?
Child support is ordinarily paid through the state child support trust, and if you've been doing that, then there should be no problem in proving that.
If you have been paying her directly, you might have canceled checks (or get scanned images of canceled checks from the bank) or have bank statements that show the designated amount.

It sounds like your wife has filed a petition to have your support payments reviewed by the court, so if you're not quite sure what to, consult with a family law attorney to help you put together the proof you need to make your case at the hearing.