Sounds like you need to do a servo reverse. On a quick glance the DXe can be programmed via a PC or your phone with a data cable that it hopefully came with and a software download from Spektrums website.
if you can find the manual it gives instructions on how to reverse things using the gimbal sticks. no cable or nothing required. you will just have to play with it to get it right for your elevons.
I forgot to get back on here. You can to simple settings like servo reverse without the software and cable per the manual. Not sure about the elevons though. Looks like that is a programming thing that has to be downloaded to the TX once the model config is set up.