If you just want to change the playback tempo and can live with one of
the MIDI file players that has that capability and don't mind changing
it every time you play the file, then do it that way.  If you want to
permanently change the tempo, you need to change the tempo event(s)
internally in the file.

Most files contain just one tempo event in the beginning and some don't
even have one, deferring to the default value.  If this is the case,
you just need to edit the file in a sequencer or other program that
lets you change the single event or insert one at the beginning of the
file.

Some files contain more than one tempo event throughout the file.  This
can be real fun to fix as just missing one, will allow the playing to
revert to the unedited tempo until another occurs.  I wrote a program
for scaling all tempos in a MIDI file by a certain percentage.  I think
it is on the download page of my web site and is called MIDI file tempo
scaler not sure if the posted file is the most current so let me know
if there are any problems with it as I probably have a newer version.

  http://www.spencerserolls.com/Files4Download.html 

There are a bunch of other MIDI file utilities that may be useful to
others.  I have many, many others that are not posted, in case anyone
has other needs.

Regarding a good reference for a MIDI newbie -- good luck!  There are
some basic tutorials online but anything beyond that gets very
technical.  It took me years to figure out most of it.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/