You don't mention which version of Finale you are using : "full", "Printmusic"...(please add some info to your signature).
You can transpose a staff :
- from the Staff attributes window (staff tool, doucle click on the staff)->Transposition. Octave down is available there.
Of course transposition will display if "Display in concert pitch" is NOT selected in the menus. Transposition will also be applied to an extracted part.
Note that in the same place, you can also select "set to clef" to choose the bass clef in the transposition.
- Using the "Transpose" function" on a select region of music. See my previous post.
Use the clef tool to change the clef to a bass clef.
Changing the sound is done in the Instrument list window. In the middle column, use the pop up menu to select New Instrument, and create a new, unused midi channel and a sound patch, in this case the trombone.
MacOsX 10.11.6, Finale (25.2), 2014.5, 2012c, Mac Mini Intel Core i7 2,7 Ghz, 16 Go Ram, Dolet 6. Azerty kb. Full TGTools. MOTU Midi Express XT USB, Roland Sound Canvas SC-88vl, MOTU Audio Express.