how to launch one channel from another & comeback to previous channel on Back button

You might be able to use Deeplinking special field to set an argument in the launch url

https://sdkdocs.roku.com/display/sdkdoc/Deep+Linking

http://<IP of Roku>:8060/launch/?special=<channel to return to ID>

You could check if this is present  and then set conditional if present on back button launch previous channel.
function main(args={})

If back_button_pressed and args.special<>invalid  then launchChannel(args.special)
end function 

Though Certification asks you to return to "Roku Home menu" if back button is pressed on top level screen. Maybe a "Dialog box" asking if user wants to return home or to previous channel