Replacing a 1991 Sentra Alternator in the parking lot...

My son's car died last night and it sounds like the alternator.
Anyway, assuming it is, I need to go down and replace it in the
parking lot where it died.  Where does this sit and what tools will I
need to get it out?   It's 24 degrees here so I want to do this as
quickly as possible.

-Jim