To separate stereo audio signal from mono audio signal how can you do that?

If you have a mono signal then all stereo parts are gone and you really cannot get them back.

The other way round is possible. You can add the left hand channel to the right hand channel and you get mono.

like the guy said just reverse the left channel to right and right to left it will create a reveresed sound A.K.A mono. but why would you want to switch anyway? stereo is the peak of all speaker systems.