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:
   COMCOD	ACTCODE	ACTDESC
    3306                190001	 ADVANCE TO STAFF
    3306   	190002	ADVANCE TO OTHERS
    3306    	190003	ADVANCE TO SITE OFFICE

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:
   COMCOD	ACTCODE	ACTDESC
    3306                190001	 ADVANCE TO STAFF
    3306   	190002	ADVANCE TO OTHERS
    3306    	190003	ADVANCE TO SITE OFFICE

UPDATE T
SET ACTCODE = STUFF(ACTCODE, 1, 3, '190')
WHERE ACTCODE LIKE '180%';