on update, how to change the value?

Hi.
If I have table A (x integer primary key);
and table B (y integer references A on delete cascade );

and that a new item (x=70) gets inserted into A and a lot of items go into B
that references the new item in A.
Now, if I really have to do:

delete from A where x=5;
update A set x=5 where x=70;

is there a way to make all the items in B change from 70 to 5 automatically
(it must be done automatically since B can be a lot more than a signle
table)?

There is an "on update" option to the create table command, but I can't
find out if that will help me. The docs are unfortunatly a little short on
describing the "on update" option.

Will "on update cascade" propagate the update to the B table?