How to Add Identity Column to Table Based on Order of Another Column

USE tempdb

GO
-- Create Table
CREATE TABLE TestTable (Col1 INT, Col2 VARCHAR(100))
GO
-- Insert Data
INSERT INTO TestTable (Col1, Col2)
VALUES (33, 'Ranjan');
INSERT INTO TestTable (Col1, Col2)
VALUES (22, 'shyam');
INSERT INTO TestTable (Col1, Col2)
VALUES (11, 'Tester');
GO
-- Select Data
SELECT *
FROM TestTable
GO
-- Add Identity Column
ALTER TABLE TestTable
ADD ID INT IDENTITY(1, 1)
GO
-- Select Data
SELECT *
FROM TestTable
GO
-- Clean up
DROP TABLE TestTable
GO

OR

USE tempdb

GO
-- Create Table
CREATE TABLE TestTable (Col1 INT, Col2 VARCHAR(100))
GO
-- Insert Data
INSERT INTO TestTable (Col1, Col2)
VALUES (33, 'Ranjan');
INSERT INTO TestTable (Col1, Col2)
VALUES (22, 'Shyam');
INSERT INTO TestTable (Col1, Col2)
VALUES (11, 'Tester');
GO
-- Select Data
SELECT *
FROM TestTable
GO
-- Create Clustered Index on Column ID
CREATE CLUSTERED INDEX IX_TestTable ON dbo.TestTable
(Col1 ASC)
GO
-- Add Identity Column
ALTER TABLE TestTable
ADD ID INT IDENTITY(1, 1)
GO
-- Select Data
SELECT *
FROM TestTable
GO
-- Clean up
DROP TABLE TestTable
GO