You can't change the channel volumes after the fact on the keyboard itself.
But if you recorded the song as a User Song rather than as an audio file, you can convert the User Song to a standard MIDI file using the "Save SMF" function or whatever it's called on the DGX, which should write the User Song to your USB thumb drive in MIDI format.
Then you can load the MIDI file into a MIDI editor or DAW and make whatever changes you want.
Afterward, you can save the changed MIDI data back to the USB thumb drive.
Then you can play back the changed MIDI song file on the DGX and record the audio output in an audio recorder or DAW.