create table blist (
  serial_no varchar(8) primary key,
  city char(2) not null
);

insert into blist 
  (serial_no, city) values
  ('abcd_086', 'US') 
, ('abcd_087', 'US') 
, ('abcd_088', 'US') 
, ('abcd_089', 'US') 
, ('abcd_090', 'US')
, ('dcba_001', 'US')
, ('abcd_999', 'HK')
;

update blist
set serial_no = replace(serial_no, 'abcd_', 'xyz_')
where city = 'US'
  and serial_no like 'abcd%';

SELECT * FROM blist;