Help in Update script

Question

Hi, How I can update the SEQUENCE column in Seq_Order column in one update script. It shoud be decrease by -1. Thanks.
Current Data of Seq_order table
SEQUENCE   SEQ_No
24               300
NULL            301
NULL            302
NULL            303
NULL            304
NULL            305
--Desired Output
SEQUENCE   SEQ_No
24               300
23              301
22             302
21             303
20             304
19            305

Answer 1

-----SQL Server 2012, 2014
create table test3 (SEQUENCE int, SEQ_No int)
insert into test3 values(24,300),(null,301),(null,302),(null,303),(null,304),(null,305)
;with mycte as (
select Sequence, mySum - sum(cnt) Over (Order by SEQ_NO)+ 1 as [SequenceNew] , SEQ_No from
select * , sum(SEQUENCE) Over (Order by SEQ_NO) mySum
, count(SEQUENCE) Over(Order by SEQ_NO) cnt from test3) t )
Update mycte
Set Sequence=[SequenceNew]
select * from test3
drop table test3