The modifications to MIDI given here for the MAC work the same for PC.
1. Open Reaper (Select reaper.app), then insert the MIDI track (Insert—Media File OR drag the file from the desktop to Reaper)
2. If it is multi-track have Reaper create multiple tracks (expand).
3. Double click on your MIDI file in Reaper to open the MIDI editor.
4. View the MIDI in the event list (View—Mode: event list)
Changing an instrument
Near the top of the MIDI file find “PC” under the “Type” column. Double click the number in the value column and enter a new General MIDI instrument number.
Changing Notes
You can alter note values in the event list, or drag notes in the piano roll (View—Mode: piano roll)
Saving Your Changes as MIDI
1. Select the tracks you wish to save.
2. File—Export project MIDI. Choose from “Entire project” or “Time selection only”—this allows you to save all the file or just a portion of it. Choose from “All” or “Selected tracks only.” Here I have only changed one track so I will only save that track. If you only have a single MIDI track choose “Merge to single MIDI track.” Otherwise choose the “Multitrack MIDI file. Save the file in a place you can retrieve it.
You’re done!