IF OBJECT_ID(N'tempdb..#T1') IS NOT NULL 
    DROP TABLE #T1;
	
IF OBJECT_ID(N'tempdb..#T2') IS NOT NULL 
    DROP TABLE #T2;
	
CREATE TABLE #T1 ([Number] VARCHAR(10));

CREATE TABLE #T2
    ([OldNumber] VARCHAR(10),
     [NewNumber] VARCHAR(10));

INSERT  INTO #T1
        (Number)
VALUES  ('10'),
        ('20'),
        ('30'),
        ('40');

INSERT  INTO #T2
        (OldNumber, NewNumber)
VALUES  ('10', '15'),
        ('20', 'MISSING'),
        ('30', NULL);

SELECT  *
FROM    #T1;

MERGE INTO #T1 AS Tgt
    USING 
        (SELECT #T2.OldNumber,
                #T2.NewNumber
         FROM   #T2) AS Src
    ON Tgt.Number = Src.OldNumber
    WHEN MATCHED AND Src.NewNumber IS NOT NULL
        AND Src.NewNumber != 'MISSING'
        THEN UPDATE
          SET       Number = Src.NewNumber;
        
SELECT  *
FROM    #T1;