Do you have the Owner’s Manual? If not, you can download it from Yamaha’s web site.
The DGX-230/YPG-235 doesn’t have a USB TO DEVICE port, so you can’t plug a USB flash drive into it for saving and loading files.
However, it does have a USB TO HOST port, so you can use a standard USB 2.x cable to connect it to a computer or iPad/iPhone/iPod touch, then you can use Yamaha’s MusicSoft Downloader program (on a Windows computer) or MusicSoft Manager app (on an iPad/iPhone/iPod touch) to save a backup of the keyboard’s memory or load a previously-saved backup into the keyboard’s memory. See pages 82 - 84 of the Owner’s Manual for more information.