Modify table constraint in PostgreSQL
To sum up the comments:

Like @ypercube hinted, you can do it in a single command, which is cheaper and safer:

ALTER TABLE distributors
  DROP CONSTRAINT zipchk
, ADD  CONSTRAINT zipchk CHECK (length(zipcode) = 6);


ALTER CONSTRAINT in Postgres 9.4 or later (like you found) can only change the "deferability" of a FK constraints. So not what you are looking for. Other than that, there is no "MODIFY (like) command" for constraints (referring to MySQL's MODIFY I assume).

Details in the manual for ALTER TABLE, like @a_horse mentioned.