If you want to change the tempo of a MIDI file, do this:
Install the lilypond program (free from lilypond.org)
Open the file you want to edit (named in the form air_N.ly using the same Roman numerals as in the text) and scroll down to the bottom. The make-moment line works just like a metronome setting. Leave the second number alone—usually a 4 for quarter-note—and make the first number larger or smaller.
Select “Run” or “Typeset File” from the Compile menu. This will create three files in the same location as the original .ly file: an updated MIDI, a new PDF, and a Postscript (.ps) file. You may keep the Postscript file or delete it; they are automatically generated, but were omitted from this Project Gutenberg text because they are very large and are easy to make on your own computer.