Hi ROHIT,

You can't alter the existing columns for identity.

You have 2 options. 1. Create a new table with identity & drop the existing table. 2. Create a new column with identity & drop the existing column.

Check the below example.

SQL

CREATE TABLE Books
(
	ID INT,
	Name VARCHAR(50)
)
INSERT INTO Books VALUES(1,'SQL Server')
INSERT INTO Books VALUES(2,'ASP.NET')
INSERT INTO Books VALUES(4,'C#')

--In this Approach you can retain the existing data values on the newly created identity column.
CREATE TABLE Tmp_Books
(
	Id int NOT NULL IDENTITY(1,1),
	Name varchar(50) NULL
)  ON [PRIMARY]
GO
SET IDENTITY_INSERT Tmp_Books ON
GO
IF EXISTS(SELECT * FROM Books)
	INSERT INTO Tmp_Books(Id, Name)
	SELECT Id,Name FROM Books TABLOCKX
GO
SET IDENTITY_INSERT Tmp_Books OFF
GO
DROP TABLE Books
GO
EXEC sp_rename 'Tmp_Books', 'Books'

-- In this approach you can't retain the existing data values on the newly created identity column.
-- The identity column will hold the sequence of number.
ALTER TABLE Books 
ADD Id_new Int Identity(1,1)
GO
ALTER TABLE Books 
DROP COLUMN ID
GO
EXEC sp_rename 'Books.Id_new', 'ID','Column'

Create a stored procedure with the query and call the stored procedure in code.

For calling stored procedure in code behind refer below article.

Select SQL Server Stored Procedures using ASP.Net Example