DateTime field stores numeric value. So, format is not important, because it's used for visualize data.
To change format of viewed date, use:
SET DATEFORMAT ymd;
SET DATEFORMAT dmy;
SET DATEFORMAT mdy;
More about:
DATETIME[
^],
SET DATEFORMAT[
^],
CAST and CONVERT[
^]
DECLARE @myDate DATETIME
SET @myDate = '2013-03-07'
SELECT CONVERT(NVARCHAR(30), @myDate, 101) AS [Formated Date]
SELECT CONVERT(NVARCHAR(30), @myDate, 103) AS [Formated Date]
SELECT CONVERT(NVARCHAR(30), @myDate, 110) AS [Formated Date]
SELECT CONVERT(NVARCHAR(30), @myDate, 111) AS [Formated Date]