how to alter/drop check contraint?

the subject should be self-explanatory, but:
i have table:
create table a (b text check (length(b)<10));
and for some reason i want to drop this check or alter this to length(b)<20.
how can i do so?

or maybe using trigger in plpgsql will be better?
how to make trigger which will stop insert or update when something occurs?

depesz

--
hubert depesz lubaczewski
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