Steps to do:
1) add new column
ALTER TABLE grimage ADD COLUMN tbbnum NUMERIC(10)
2) copy data to new column
UPDATE grimage SET grimage.tbbnum = t1.tbbnum
FROM grimage INNER JOIN
(
SELECT CONVERT(NUMERIC(10), t1.tbb) AS tbbnum FROM grimage
) AS t1 ON grimage.id = t1.id
3) remove unused column
ALTER TABLE grimage DROP COLUMN tbb
That's all!