If you're using the standalone version (simply put, not playing it in RetroArch) Just go to the Settings and the first setting is Video Format I think it was. You can choose between Progressive, Interlaced and Original.
Choose Original and games will play at 240p. Personally I like using GenplusGX on Retroarch because when correctly set up it has a better picture quality, but the standalone version is less fuss for most people.