Procedure for updating records meeting specific conditions

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SQL

SQL-Server-2012

Hi All
Stuck up with a Sql Stored Procedure to update a specific column in Sql server. I need to update the "CurCustType" to "R" for all those Customer who purchases regularly across any five days in a week.

What I have tried:

The query :-

Create Procedure ModifyCustType
As 
BEGIN
while (Count(CustName)>=1)

Update [CustmrSrc] set CurCstType = 'R'

where  Exists

(SELECT * FROM [CustmrSrc]  a
JOIN (SELECT CustName FROM [CustmrSrc]
GROUP BY CUSTName, DATEPART(ISO_WEEK,PurchaseDate)
HAVING COUNT(PurchaseDate) >= 5) b
ON a.CustName = b.CustName)

END

Its ending up with error: "invalid column name 'CustName'"
What i could infer from this is there is something wrong with the while condition i tried, but i was blank on the specific condition to be applied here.Need help on this.

Regards