How to update Sequence Column

select t.rowno, t.EmpID, t.Salary into #temp from
(
   select row_number() over(partition by EMPID order by EmpID) as rowno, empID, Salary
   from ActualTable
) as t

-- to see the result you can do
select * from #temp

-- update your data
update ActualTable
 set Sequence = t.rowno
from #temp as t
inner join ActualTable as a on a.empID = t.empID and a.Salary = t.Salary

drop table #temp