Download or purchase MIDI-editing software. Some programs specifically function as MIDI editors, but you can also use digital production programs such as GarageBand, Pro Tools and Logic.
Open your editing software. If a new project window appears, proceed to the next step. If no project window appears, click "File" at the top of your screen and select "New."
Click "File" followed by "Import" on your menu bar. In some cases, the menu may read "Import MIDI" or "Upload MIDI." Some programs will also allow you to click the MIDI file on your desktop and drag it directly onto the project window for an automatic import.
Click any MIDI note on your screen to highlight it. When you import your MIDI file, it will appear as a horizontal row of tiny dots or squares arranged on a grid. Moving from left to right, they represent the time line of the song or note sequence. If a note appears near the top of the grid, it corresponds to a high note, and if it appears near the bottom of the grid, it corresponds to a low note.
Drag your highlighted note with your mouse. You can change the note to a higher or lower value by moving it up or down, or move it to a different part of the time line by dragging it left or right. For example, if the note currently rests at "C," you can change it to a "D" note by moving it two spaces up on the grid (from "C" to "C#" to "D"). Move as many notes as you want to edit.
Click the "Delete" key to remove a highlighted note from the grid completely. In some cases, you may want to remove individual notes or entire note sequences from the MIDI song file.
Open your "Voices," "Instruments" or "Software Synthesizers" menu to change the instrument sound of your MIDI sequence. You may find your instrument options on your menu bar or directly on the project window (depending on the software). For example, if you have the "Happy Birthday" melody and you select "Trumpet" from your Instruments menu, you can hear the song "Happy Birthday" played with trumpet sounds. Choose any instrument that works for your particular sequence.
Save your work. After editing, you can click "File" followed by "Save" on your menu bar to save the finished product. To export it as a new MIDI (".mid") file, click "File" followed by "Export."