Update records in table from CTE

I have the following CTE that will give me the DocTotal for the entire invoice.

 ;WITH CTE_DocTotal
 AS
 (
   SELECT SUM(Sale + VAT) AS DocTotal
   FROM PEDI_InvoiceDetail
   GROUP BY InvoiceNumber
 )

UPDATE PEDI_InvoiceDetail
SET DocTotal = CTE_DocTotal.DocTotal

Now with this result I want to enter into the column the DocTotal value inside PEDI_InvoiceDetail.

I know is not going to work and I know I am missing something, what is it?

Updates you make to the CTE will be cascaded to the source table.

I have had to guess at your schema slightly, but something like this should work.

;WITH T AS
(   SELECT  InvoiceNumber, 
            DocTotal, 
            SUM(Sale + VAT) OVER(PARTITION BY InvoiceNumber) AS NewDocTotal
    FROM    PEDI_InvoiceDetail
)
UPDATE  T
SET     DocTotal = NewDocTotal

WITH CTE_DocTotal (DocTotal, InvoiceNumber)
AS
(
    SELECT  InvoiceNumber,
            SUM(Sale + VAT) AS DocTotal
    FROM    PEDI_InvoiceDetail
    GROUP BY InvoiceNumber
)    
UPDATE PEDI_InvoiceDetail
SET PEDI_InvoiceDetail.DocTotal = CTE_DocTotal.DocTotal
FROM CTE_DocTotal
INNER JOIN PEDI_InvoiceDetail ON ...

You don't need a CTE for this

UPDATE PEDI_InvoiceDetail
SET
    DocTotal = v.DocTotal
FROM
     PEDI_InvoiceDetail
inner join 
(
   SELECT InvoiceNumber, SUM(Sale + VAT) AS DocTotal
   FROM PEDI_InvoiceDetail
   GROUP BY InvoiceNumber
) v
   ON PEDI_InvoiceDetail.InvoiceNumber = v.InvoiceNumber