Fix twocheckout integration or revert back to original files

I am trying to integrate 2checkout with OpenCart. I followed the instructions here (https://www.2checkout.com/shopping-cart/open-cart) for integrating the API. I downloaded the files from Github and uploaded them. But now when I go to Extensions->Payments->2Checkout to edit, I get a bunch pf php notices like this...

Notice: Indirect modification of overloaded property ControllerPaymentTwoCheckout::$data has no effect in /hermes/bosnaweb18a/b2928/sl.megachomplurecocom/public_html/admin/controller/payment/twocheckout.php on line 20

I contacted 2checkout and they told me to contact OpenCart.

What went wrong?
How can I replace the twocheckout files I uploaded with the original ones installed by OpenCart?

Thanks in advance