If by Factory Reset, you mean all user presets removed AND back to the factory-installed firmware, you need to reflash the firmware using Monkey.
If you just want it back to the factory tone presets, but keeping the latest-installed firmware (which is what you SHOULD do if you're selling it), simply unplug it, hold down the left arrow, and plug it in, continuing to hold down the left arrow until the A1 preset appears on the screen.