Performance tuning a SQL query

Hello ,

I have here t-SQL query  that is executed very frequently and taking long duration  I was asking what is place I have to hit to tune this query

UPDATE B
SET B.ID = CASE ISNULL(CU.is_Status, 1) WHEN 1 THEN 1 WHEN 3 THEN 2 WHEN 2 THEN 1 END,
 B.ID = A.ID
FROM table1 A 
INNER JOIN table2 CS
ON A.Status = CS.ID
INNER JOIN Customers cu
ON A.Customer_Id = cu.Id
INNER JOIN linkedsrv.dbname.DBO.table2 B
ON A.PAN COLLATE database_default=B.ID_2 AND 
(
	(B.ID <>CASE ISNULL(CU.Status, 1) WHEN 1 THEN 1 WHEN 3 THEN 2 WHEN 2 THEN 1 END)
	OR (B.ID <> A.ID)
)
WHERE 
A.@11 = @12
AND left(a.CUID,7) IN
(
	SELECT b.name
	FROM dbo.table3 AS a
	INNER JOIN dbo.table4 AS b
	ON a.id_001= b.id
	WHERE a.id_002=2
)