adding on delete cascade constraint?

Currently I have a table defined as such:

TAL=# \d internal_keywords
Table "public.internal_keywords"
Column | Type | Modifiers
---------+---------+-----------
keyword | text | not null
pid | integer | not null
Indexes: internal_keywords_pkey primary key btree (keyword, pid)
Foreign Key constraints: $1 FOREIGN KEY (pid) REFERENCES products(id) ON
UPDATE NO ACTION ON DELETE NO ACTION

How can I change the ON DELETE action to CASCADE for column pid?

I've check the alter table documentation but cannot find any reference
to this.

Thanks,

Jean-Christian Imbeault