Set identity column - existing table

Create TABLE DataTable(id int, name varchar(20) )
Insert into DataTable(id, name)
Values(1, 'test 0'),
(4, 'test 1'),
(5, 'test 2'),
(6, 'test 3')
GO

--Create a temp table with an identity column
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Tmp_DataTable
(
id int NOT NULL IDENTITY (1, 1),
name varchar(20) NULL
) ON [PRIMARY]
GO
SET IDENTITY_INSERT dbo.Tmp_DataTable ON
GO
IF EXISTS (SELECT 1 FROM dbo.DataTable)
INSERT INTO dbo.Tmp_DataTable (id, name)
SELECT id, name FROM dbo.DataTable WITH (HOLDLOCK TABLOCKX)
GO
SET IDENTITY_INSERT dbo.Tmp_DataTable OFF
GO
DROP TABLE dbo.DataTable
GO
EXECUTE sp_rename N'dbo.Tmp_DataTable', N'DataTable', 'OBJECT'
GO
COMMIT

select * from DataTable

insert into datatable(name)values('arun')