Hi americo, I'm not sure I 100% understand, but to start with you probably want to extract the "underlying surface" for the upper surface which you can do by selecting that face:



Then run Edit > Separate to break that face off from the other ones so it is independent.

Then select one of that surface's edges, then do Ctrl+A which will select all of its edges and then hit the Delete key to remove the trim curves and recover the full underlying surface there, that will look like this:



So that now gives you a broad surface to work with as the base.


Then you may want to try and model the bowed out part as a separate surface, and cut some open area between that part and this surface and put in a blend, something similar to this technique:










I hope this gives you some ideas!

- Michael