You first need to delete the original Form 1099-B.  Instead of over writing the the first Form 1099, delete it and then repeat the same process you used to import the first one.  To do so, click on "Search", type Form 1099-b in the search window and hit Enter.  Click on "Jump to Form 1099-b".   Click on "Delete" next to the broker you need to delete the 1099 for.