How do I fit the foot pedal back on on a SEBO

This is much easier to do than to describe! Your question suggests that you plan to re-fit the same pedal that has come off, so first you need to be sure that nothing actually broke when it came off. You should be able to clip the plastic part back on to the metal part - it should stay there and not fall off (if it falls off, you need a new one). The trick is to get the flat metal part correctly located in the plastic part, so with the plastic part not clipped on, pull the metal part down and rest it in the plastic part, then push the plastic part in to position. The flat metal part should then be trapped behind two projections on the plastic part, so that there is some movement, but not much. These projections may possibly have worn away, allowing the pedal to come away in the first place, in which case you will need a new piece.
I hope this helps. If so, please remember to rate this response as I value feedback. If you need more help, just post a reply here and I'll get back to you as soon as I can. Good luck!