IF (OBJECT_ID('tempdb..#test') IS NOT NULL)
   DROP TABLE #test
CREATE TABLE #test (id INT, txt VARCHAR(100))

-- Some test data
INSERT INTO #test
   SELECT TOP(100) NULL, name
   FROM master.sys.columns


-- Create an incremental id
DECLARE @id INT
SET @id = 100

UPDATE #test SET @id = id = @id + 1

-- Result
SELECT * FROM #test