You don’t edit or modify the mesh in Fusion in any way. You use it purely as a reference to do section sketches.
There is a quick and dirty way to get an editable solid though in Fusion.
Follow my guide in this post:
You use a program call Instand Meshes to create a quadmesh, this you can then convert in to a solid in Fusion that is nicer than a faceted solid you get from a normal input.