this is the modified one as per suggested and now i get ora-00927 missing equal sign

update temp_ts
set start_time = decode(start_time,null, to_date(to_char(rk_date,'dd-mon-yyyy')) || ' 8:00:00 AM','dd-mon-yyyy hh:mi:ss am'),to_date(to_char(st_time,'dd-mon-yyyy')) || ' 8:00:00 AM','dd-mon-yyyy hh:mi:ss am')),
it_time = decode(it_time,null,to_date(to_char(rk_date,'dd-mon-yyyy')) || ' 8:00:00 AM','dd-mon-yyyy hh:mi:ss am'),
to_date(to_char(it_time,'dd-mon-yyyy')) || ' 8:00:00 AM','dd-mon-yyyy hh:mi:ss am'))
where hours = 0 and break_time = 0