IF OBJECT_ID('tempdb..#Source') IS NOT NULL
    DROP TABLE #Source
IF OBJECT_ID('tempdb..#Target') IS NOT NULL
    DROP TABLE #Target

CREATE TABLE #Source ( Id INT, Value CHAR(2) )
CREATE TABLE #Target ( Id INT, Value CHAR(2) )

INSERT  INTO #Source
        ( Id, Value )
        SELECT  1 ,
                'a'
        UNION ALL
        SELECT  2 ,
                'b'

INSERT  INTO #Target
        ( Id, Value )
        SELECT  1 ,
                'a1'

SELECT * FROM #Source
SELECT * FROM #Target

MERGE [#Target] AS Target
   USING [#Source] AS Source
     ON Target.Id = Source.Id
WHEN MATCHED AND
    (
         COALESCE(Target.Value, '') <> COALESCE(Source.Value, '')
    )
    THEN UPDATE SET
       Target.Value = Source.Value
WHEN NOT MATCHED
    THEN INSERT (
        Id
       ,Value
      ) VALUES (
        Source.Id
       ,Source.Value
      );

SELECT * FROM #Source
SELECT * FROM #Target