Rename enum item in PostgreSQL

The query in the accepted answer doesn't take into account schema names. Here's a safer (and simpler) one, based on http://tech.valgog.com/2010/08/alter-enum-in-postgresql.html

UPDATE pg_catalog.pg_enum
SET enumlabel = 'NEW_LABEL'
WHERE enumtypid = 'SCHEMA_NAME.ENUM_NAME'::regtype::oid AND enumlabel = 'OLD_LABEL'
RETURNING enumlabel;

Note that this requires the "rolcatupdate" (Update catalog directly) permission - even being a superuser is not enough.

It seems that updating the catalog directly is still the only way as of PostgreSQL 9.3.

In PostgreSQL version 10, the ability to rename the labels of an enum has been added as part of the ALTER TYPE syntax:

ALTER TYPE name RENAME VALUE 'existing_enum_value' TO 'new_enum_value'

Update: For PostgreSQL version 10 or later, see the top-voted answer.

Names of enum values are called labels, attributes are something different entirely.

Unfortunately changing enum labels is not simple, you have to muck with the system catalog: http://www.postgresql.org/docs/9.1/static/catalog-pg-enum.html

UPDATE pg_enum SET enumlabel = 'Aborted' 
WHERE enumlabel = 'Task created' AND enumtypid = (
  SELECT oid FROM pg_type WHERE typname = 'import_action'
)