with cte as (
Select *,datePart(WEEKDAY,Dt) as WeekDay,
PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY SaleCount) Over (partition by ItemRelation, DocumentNum, DocumentYear) as PERCENTILE,
avg(SaleCount) over (Partition by ItemRelation, DocumentNum, DocumentYear,datePart(WEEKDAY,Dt), IsPromo) as AVG_WeekDay
From promo_data)
Update a 
Set SaleCount = cte.AVG_WeekDay
From CTE
join promo_data a 
  on a.Dt = cte.dt
 and a.ItemRelation=cte.ItemRelation 
 and a.DocumentNum = cte.DocumentNum 
 and a.DocumentYear = cte.DocumentYear 
 and a.ispromo = cte.ispromo
Where CTE.PERCENTILE < CTE.SaleCount
and datePart(WEEKDAY,CTE.Dt) < 5
and CTE.ispromo = 0 ;