Add or Modify IDENTITY property to an existing column

Method 1
Easiest method is to use SSMS to change the property.

• Goto SQL Server Management Studio
• Right click on the table name
• Choose Design/Modify
• Choose the column which needs to be set as Identity
• At the bottom window (Column Properties) for that column you can see "Identity Specification" has been by default set to "NO".
• Just change it to "YES" and save the table (CTRL + S).

Method 2

If there is no data in the table then use this method.

Drop the already exist column 

ALTER TABLE TempTable DROP Column ID;

GO;

Now add the ID column again with Identity specification on.

Alter Table TempTable Add ID INT IDENTITY
GO 

Method 3

If there is data in the table then use this method.

• Create a new table with identity column
• Enable IDENTITY_INSERT for this new table
• Move the data from this old table to this new table.
• Disable IDENTITY_INSERT for this new table
• Delete the old table.
• Rename the newtable with the oldtable name.