I too, just recently purchased the M5 Stompbox and was attempting to change the factory loaded 24 presets with other box FX with no luck. Until I realized that even when the newly selected preset is loaded the display name does not change. You have to manually type in the name of the new preset. Then once you press and hold the Model Select knob the new preset saves into whichever of the 24 presets you are changing.
So to change preset FX enter preset mode by simultaneously pressing the On/Off and TAP switches, and select one of the 24 presets. Press On/Off and TAP switches again to return to Performance Mode. Select the specific FX and then Press and hold the Model Select knob until the save display appears. Then, using the knobs spell the name of the selected FX. Once complete press and hold the Model Select knob. Now the selected effect is saved to that numbered preset. Hope this helps.