Something like this?

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