If I understand what you desire to do correctly, this should do the job.

CREATE TABLE T(COMCOD nchar(4), ACTCODE nvarchar(6), ACTDESC nvarchar(250),

PRIMARY KEY (ACTCODE))

INSERT INTO T

SELECT '3306', '180001','ADVANCE TO STAFF' UNION ALL

SELECT '3306', '180002','ADVANCE TO OTHERS' UNION ALL

SELECT '3306', '180003','ADVANCE TO SITE OFFICE'

UPDATE T SET ACTCODE = '190' + SUBSTRING(ACTCODE,4,3)

WHERE SUBSTRING(ACTCODE,1,3) = '180'

Results:

COMCODACTCODEACTDESC

3306 190001 ADVANCE TO STAFF

3306 190002ADVANCE TO OTHERS

3306 190003ADVANCE TO SITE OFFICE

Remember to test, and then test again before using in a Production database