Newbie Flip OBJ file on blender

Hey there, can someone help me plz, I import obj to blender and flip it with ctrl + M then export it to obj again but the result is some part of that file is become black

how to fix it ?

Did you try to recalculate the Normals after the flip? In Edit Mode, check under Mesh->Normals menu, there are Recalculate options.