If you have a constraint you wish to alter, you need to drop it and re-add it.
First let's look at the constraint with
\d+ youtable ... Foreign-key constraints: "constraint_name" FOREIGN KEY (some_id) REFERENCES yourtable(some_id)
We can we see the, in this case, foreign key constraint we want to alter. We'll use text above later.
Now let's drop the constraint.
alter table yourtable drop constraint constraint_name;
Now let's copy the text we saw above, and paste it after the text
alter table yourtable add constraint.
alter table yourtable add constraint "constraint_name" FOREIGN KEY (some_id) REFERENCES yourtable(some_id) on delete cascade;
In our case, we altered the constraint by adding
on delete cascade.