Thank you first of all. Yes; I understood that the scenes are presets and that I can put up to 4 effects in one scene. But I thought, that the M13 has two different operaion modes: a stompbox mode and a scene mode. In the stompbox mode you can assign one stompbox effect to each of the 12 slots and you can use 4 at the same time, one from each FX Unit. I've worked with this function so far. Foe example I assigned the Wah Wah to 1A, Compressor to 1B etc, all 12 slots.
If I press the scenes button now, the M13 shows the names of the 12 scenes and I can choose one. If I want another one I press the scenes button again and pick another one (momentary scenes mode). Works fine. But there seems to be no way to get back to the point, where I had my 12 effects assigned to the slots. In other words, I can't stop the scene mode and returen to the stompbox mode. I hope you understood my problem, because my English is not that perfect.