You can change instruments for a Midi file on Apple’s Garageband software for your Mac. Follow the simple steps below

Get your midi file.

Open GarageBand.

Put your midi file into GarageBand.

It will automatically separate the track into instruments that could be customized

Select a track.

Go down to the bottom right. Press the button that has a circle around an “i”.

Select your instrument.

Play your track.

Note

After you’re done, you can go to the Share menu, and click “Send Song to iTunes”.