If you have the game running, first close it.
Navigate to your game installation folder, and open the MX vs ATV Reflex folder.
For example, C:\Program Files (x86)\GameSessions\MX vs ATV Reflex.
Run the program "MXSettings.exe".
Chose the resolution you wish to use, as well as any other setting you wish to change, click on "OK", and run the game.