How to change the auto numbering id field to seria

I have a Database which is migrated from MSSQL to PostgreSQL(9.2).
This Database have 100+ tables, These table have autonumbering filed(PRIMARY KEY field), given below is an example for a table

CREATE TABLE company
(
  companyid integer NOT NULL DEFAULT nextval('seq_company_id'::regclass),
  company character varying(100),
  add1 character varying(100),
  add2 character varying(100),
  add3 character varying(100),
  phoneoff character varying(30),
  phoneres character varying(30)
  CONSTRAINT gcompany_pkey PRIMARY KEY (companyid)
)

sample data

 INSERT INTO company (company, add1, add2, add3, phoneoff, phoneres) VALUES   
 ('company1','add1','add2','add3','00055544','7788848');
 INSERT INTO company (company, add1, add2, add3, phoneoff, phoneres) VALUES  
 ('company2','add9','add5','add2','00088844','7458844');
 INSERT INTO company (company, add1, add2, add3, phoneoff, phoneres) VALUES 
 ('company5','add5','add8','add7','00099944','2218844');

and below is the sequence for this table

CREATE SEQUENCE seq_company_id
 INCREMENT 1
 MINVALUE 1
 MAXVALUE 9223372036854775807
 START 1
 CACHE 1;
ALTER TABLE seq_company_id
OWNER TO postgres;

while reading PostgreSQL Documentation i read about Serial Types so i wish to change all the existing auto numbering fields to serial. How to do it?

i have tried

alter table company alter column companyid type serial

 ERROR:  type "serial" does not exist
     ********** Error **********

There is indeed no data type serial. It is just a shorthand notation for a default value populated from sequence (see the manual for details), essentially what you have now.

The only difference between your setup and a column defined as serial is that there is a link between the sequence and the column, which you can define manually as well:

alter sequence seq_gcompany_id owned by company.companyid;

With that link in place you can no longer distinguish your column from a column initially defined as serial. What this change does, is that the sequence will automatically be dropped if the table (or the column) is dropped that uses it.