You can't edit a sound in preset mode, but you can load a preset, then step on both buttons to exit preset MODE. (your preset parameters remain the same as in the preset, but they are now editable.) Tweak to your heart's content. Then just save your edited preset when you finish tweaking it. Preset mode is not editable but presets are. That's kind of the point. You wouldn't want to bump a knob while you're playing and completely change the tone mid song. Think of preset mode as safe mode. I really think Line6 knocked it out of the park with POD express. No, It's not a HELIX, but it's a great sounding, versatile, fun little box at a good price point.
