CREATE TABLE tbl_PersonalDetail(ID INT IDENTITY ,[Date] nvarchar(20), Name nvarchar(20), GenderID int);

INSERT INTO Tbl_PersonalDetail VALUES(N'18-4-2015', N'Monay', 2),

                           (N'31-3-2015', N'Monay', 2),
                           (N'28-12-2015', N'Monay', 2),
                           (N'19-4-2015', N'Monay', 2)

DECLARE @Date Nvarchar(200)

SET @Date = (SELECT [Date] FROM Tbl_PersonalDetail WHERE ID = 2)

Update Tbl_PersonalDetail SET [Date] = (REPLACE(@Date , '-','/')) WHERE ID = 2

CREATE TABLE tbl_PersonalDetail
(ID INT IDENTITY ,[Date] nvarchar(20), Name nvarchar(20), GenderID int);

INSERT INTO Tbl_PersonalDetail VALUES(N'18-4-2015', N'Monay', 2),
                                     (N'31-3-2015', N'Monay', 2),
                                     (N'28-12-2015', N'Monay', 2),
                                     (N'19-4-2015', N'Monay', 2)

DECLARE @Date Nvarchar(200)

SET @Date = (SELECT [Date] FROM Tbl_PersonalDetail WHERE ID = 2) 

Update Tbl_PersonalDetail SET [Date] = (REPLACE(@Date , '-','/')) WHERE ID = 2