SQL, IF inside update clause

I'd like to evaluate a previous value inside an update clause. How can I fix this code? Thanks in advance.

MERGE INTO A
USING (select * from B ) 
ON (B.mac = A.mac)   
WHEN MATCHED THEN
update      
set A.ID = B.ID,                
IF (A.serial <> B.serial)
THEN A.valid = 0
END IF;                         
A.serial = B.serial
.....