Hi Emmanuel,
I can see nearly nothing on this small image. With the Subdivision Surface [HyperNURBS] off it would be perhaps simpler, to see something (as it seems to fold over the cut?).
The best way is normally a scene file, always reduced to the minimum.
As an alternative, you could select the edges and/or (based on your version) use then Mesh> Create Tools> Edge Cut. Then the Slide Tool to move the new edge (N-Gons: off)
My best wishes.