How to set identity column to created table in SQL server

I created a table in SQL Server like this:

CREATE TABLE [UserName]
(
   [ID] [int] NOT NULL ,
   [Name] [nvarchar] (50) NOT NULL ,
   [Address] [nvarchar] (200) NULL

   CONSTRAINT [PK_UserName] PRIMARY KEY CLUSTERED ([ID] ASC)
) ON [PRIMARY]
GO

If I want to make ID an identity column, what do I need? Do I need to drop and create this table and set ID to [ID] [int] IDENTITY(1,1) NOT NULL .

By using drop and create, all of data from UserName table are lost .

Is there another way to set IDENTITY COLUMN to created table's column without losing data?

I use SQL Server 2008 R2 :)