A lot of what you ask depends on the way the MIDI file was put together. If the melody is a separate 'track', fine - you can mute it.

Otherwise you will need to be more adventurous:

Select the track with the accompaniment and melody in it.

Copy the track to another track (paste as one track).

Carefully go through the track, erasing the melody notes. Now you have the accompaniment - which you could paste into a new file.

What you can do depends on what you have to start with!!

I'm still using Version 3 - maybe newer versions have more clever bits?