How do I drop or change a foreign key?

After I created my tables, I went back and added foreign keys to them.  Now I
need to change one of the foreign keys to 'ON DELETE CASCADE' behaviour so that
related records are automatically deleted, rather than causing an error when a
record in the parent table is deleted.  I know the name of the foreign key, but
none of the standard 'drop' commands seem to apply to it.  Am I forced to move
the data to a temp table, drop the table and recreate, then move the data back?
Improvements to the ALTER commands are already high on my personal list of
desired improvements; I run into this sort of thing fairly consistently as users
change their minds or clarify their requirements.

Thanks,

Wes Sheldahl