UPDATE x SET [APrice]= z.APrice
FROM TargetTable x
INNER JOIN 
(SELECT [ArtNo], MAX([Date]) AS MaxDate 
 FROM x
 GROUP BY [ArtNo]) y ON x.[ArtNo] = y.[ArtNo] AND y.MaxDate  = x.[Date]
 INNER JOIN SourceTable z ON x.[ArtNo] = z.[ArtNo]

UPDATE tt
SET [APrice]= st.APrice
FROM TargetTable tt
INNER JOIN 
(SELECT x2.[ArtNo], MAX(x2.[Date]) AS MaxDate 
 FROM x2
 INNER JOIN SourceTable st2 ON x2.[ArtNo] = st2.[ArtNo]
 GROUP BY x2.[ArtNo]
) AS y ON tt.[ArtNo] = y.[ArtNo] AND tt.[Date] = y.MaxDate
INNER JOIN SourceTable st ON x.[ArtNo] = st.[ArtNo]

UPDATE c SET
	APrice = z.Price
FROM
	(
	SELECT
		ArtNo,
		APrice,
		ROW_NUMBER() OVER (PARTITION BY ArtNo ORDER BY [Date] DESC) AS RN
	FROM
		TargetTable
	)  c
	INNER JOIN SourceTable z ON 
		z.ArtNo = c.ArtNo
WHERE
	RN = 1;