Assuming you’re using the factory settings on the Octave Lines page of Engraving Options, you can make Dorico display “8vb” instead of “8ba” by choosing Library > Music Symbols, choosing the Octave Lines category, finding the Ottava bassa (ba) symbol in the list, and then editing that one. On the right-hand side, choose Octaves (U+E510–U+E51F) from the list of categories, and you’ll see the “8vb” symbol in the grid below. Select that and click Add Glyph. Select the “8ba” symbol that’s also now shown in the preview next to the desired “8vb”, and click the trash can to delete it. Confirm the dialog, and that should be it.