Alter Table Auto_Increment

I have a table in my database and would like to modify the one column
that is already configured to be the PRIMARY KEY but I forgot to set
it for AUTO_INCREMENT. For some reason I can't find what the proper
command would be in the documentation and my commands from MySQL don't
appear to work properly in PostgreSQL:

sun=# \d blades
Table "public.blades"
Column | Type | Modifiers
----------+-----------------------+-----------
id | integer | not null
ilom_ip | character varying(15) |
host_os | character varying(50) |
host_ip | character varying(15) |
hostname | character varying(50) |
serial | character varying(30) |
gfe | character varying(10) |
admin | character varying(50) |
Indexes:
"blades_pkey" PRIMARY KEY, btree (id)

My command is not working so I don't know what I am doing wrong:

sun=# ALTER TABLE blades MODIFY int AUTO_INCREMENT;
ERROR: syntax error at or near "MODIFY"
LINE 1: ALTER TABLE blades MODIFY int AUTO_INCREMENT;