A MIDI editor such as Cakewalk or Powertracks allows you to change the
tempi within MIDI files.  If a file has only one tempo for the whole
file, the change is trivial.  If it has several changes, each tempo
change must be edited.  If it contains tempo ramps, the editing becomes
quite tedious.

For files that contain numerous tempo changes, I have adapted a program
I put together to change all the tempo events in a file by a percentage,
either up or down.  This is particularly useful for files digitized
from rolls where the digitizing tempo was too fast or slow.  Contact me
privately if this is what you need.

Craig Williams