Postgresql 9.4, Make existing primary key as SERIAL

Question

I am using postgresql 9.4. I want to change existing primary key with serial. My query is not working. Anybody know how to do this?

Alter table 'table_name' alter column id BIGSERIAL;

There should be a single query to modify a particular column. I didn't see that

Answer 1

CREATE SEQUENCE table_name_id_seq
   OWNED BY table_name.id;

ALTER TABLE table_name
   ALTER id
      SET DEFAULT nextval('table_name_id_seq'::regclass);