How to delete an enum type value in postgres?

You delete (drop) enum types like any other type, with DROP TYPE:

DROP TYPE admin_level1;

---

Is it possible you’re actually asking about how to remove an individual value from an enum type? If so, you can’t. It’s not supported:

Although enum types are primarily intended for static sets of values, there is support for adding new values to an existing enum type, and for renaming values (see ALTER TYPE). Existing values cannot be removed from an enum type, nor can the sort ordering of such values be changed, short of dropping and re-creating the enum type.

You must create a new type without the value, convert all existing uses of the old type to use the new type, then drop the old type.

E.g.

CREATE TYPE admin_level1 AS ENUM ('classifier', 'moderator');

CREATE TABLE blah (
    user_id integer primary key,
    power admin_level1 not null
);

INSERT INTO blah(user_id, power) VALUES (1, 'moderator'), (10, 'classifier');

ALTER TYPE admin_level1 ADD VALUE 'god';

INSERT INTO blah(user_id, power) VALUES (42, 'god');

-- .... oops, maybe that was a bad idea

CREATE TYPE admin_level1_new AS ENUM ('classifier', 'moderator');

-- Remove values that won't be compatible with new definition
-- You don't have to delete, you might update instead
DELETE FROM blah WHERE power="god";

-- Convert to new type, casting via text representation
ALTER TABLE blah 
  ALTER COLUMN power TYPE admin_level1_new 
    USING (power::text::admin_level1_new);

-- and swap the types
DROP TYPE admin_level1;

ALTER TYPE admin_level1_new RENAME TO admin_level1;