How to recreate primary after dropping primary key column?

How to recreate primary after dropping primary key column?

I would like to recreate a primary key column in pg 8.4. But the query i
am trying does not work ( it does not execute actually):
update beta
set id=rown
from
(select row_number() as rown
from beta as b over (order by b.id) -- b.id is null on all rows
) q;