For a polysurface such as your example there is no simple way to change the polysurface into a single surface. I assume your polysurface was the result of the MeshToNURB command. The results of that command are usually not very useful.
One method to create a surface from a mesh is to first create a set of curves on the mesh. Contour and, Section are the easiest way to create the curves. Curves can be drawn on the mesh. Use InterpCrv with the vertex OSnap on to get the shape of the desired curve, then use Pull to pull the curve onto the mesh. Sometimes it is useful to create a temporary set of planes and/or surface and Intersect with temporary surfaces or planes with the mesh. The location and arrangement of the curves will depend on the shape of the mesh and how the surfaces will be created. So knowledge of how to create surfaces using curves is needed.
The curves created directly from the mesh will actually by polylines with straight segments. Smooth curves, are needed to create smooth surfaces. FitCrv, Rebuild, RebuildCrvNonuniform can be used to change the polylines into smooth curves.
Once the set of curves is finished the surface or surfaces can be created. Sometimes a single surface is not desirable or even possible so a polysurface will be needed, However the polysurface will have only a few surfaces compared to the results of MeshToNURB. Commands for creating surfaces from curves include Loft, EdgeSrf, Sweep1, Sweep2 and NetworkSrf. Each has requirements on the arrangement of curves.