How can I revise multiple stock sales at once to indicate that Box 1b of my Form 1099-B is 'blank' for all of them?

I imported stock sales from Apex Clearing, but many lines with the acquisition date left blank (Box 1b) were not imported correctly. They keep getting flagged, but it takes many clicks to indicate "-Blank-" for each of them. How can I revise them all at once?