select t.id, t.sum1, tot_sum=sum(t.sum1) over (order by t.id)
from #t t

UPDATE t
set tot_sum = a.tot_sum
FROM #t t
inner join (
select top 100 percent a.id, max(a.sum1) as sum1, sum(isnull(b.sum1, 0))+max(a.sum1) as tot_sum
from #t a
left outer join #t b on b.id < a.id
group by a.id
order by a.id   --<<<<=============
) as a on t.ID = a.ID 

create table #t(id int, sum1 decimal(10,2), tot_sum decimal(10,2))

insert #t(id, sum1) values(2, 1.2)	--3 строка
insert #t(id, sum1) values(4, 1.5)	--2
insert #t(id, sum1) values(3, 0.7)	--4
insert #t(id, sum1) values(1, 2.5)	--1

 select TOP(select 100) PERCENT a.id, max(a.sum1) as sum1, sum(isnull(b.sum1, 0))+max(a.sum1) as tot_sum
from #t a
left outer join #t b on b.id < a.id
group by a.id
order by sum1 desc


UPDATE t
set tot_sum = a.tot_sum
FROM #t t
inner join (
select TOP(100) PERCENT a.id, max(a.sum1) as sum1, sum(isnull(b.sum1, 0))+max(a.sum1) as tot_sum
from #t a
left outer join #t b on b.id < a.id
group by a.id
order by sum1 desc
) as a on t.ID = a.ID 

select * FROM #t order by sum1 desc

 drop table #t

UPDATE b
set tot_sum = (select sum(a.sum1) from #t a where a.id < b.id)
FROM #t b

update #t
set
 col = value
with(index(ix_#t)) -- Используем индекс, что бы update шел в нужной последовательности