DECLARE @tbl TABLE
(
 DATE DATETIME, OrderNmbr INT, Transtype VARCHAR(10), Amount INT, InvoiceinPer CHAR(1)
)
--/*
INSERT INTO @tbl
SELECT '12/1/2010', 12345 ,'Inv', 1000 ,'N' UNION ALL
SELECT '12/3/2010', 12345 ,'Credit', -1000 ,'N' UNION ALL
SELECT '12/3/2010', 12345 ,'Rebill', 2000 ,'N'
--*/
/*
insert into @tbl
select '12/1/2010', 12345 ,'Inv', 1000 ,'N' union all
select '12/1/2010', 12345 ,'Credit', -1000 ,'N' union all
select '12/1/2010', 12345 ,'Rebill', 2000 ,'N'
*/
;WITH cte AS
(
	SELECT TOP 1 DATE FROM @tbl t ORDER BY DATE
)
UPDATE @tbl 
SET InvoiceinPer='Y'
FROM @tbl t
INNER JOIN cte ON t.date = cte.date

SELECT  *
FROM    @tbl