I'm using CGAL Surface Mesh Simplification to simplify 3d mesh from .off file was read as Linear_cell_complex_for_combinatorial_map and undo the edges collapsing (simplification) using undo_edge_collapse_surface_mesh.

How to do the simplification and undo processes within specific selected region on the mesh.

any recommendations ,please?

You can constrain some edges such that they will not be removed by passing an edge <-> bool map in argument.

See the named parameter is_constrained_edge_map in the documentation of the edge_collapse() function and this CGAL example for an example of usage. In your case, you would just loop over the edges of your regions and constrain them.