After selecting the region (and "select active layer only" if this only concerns a specific layer), try menu Utilities->Stem direction.
Michel Finale 2012c, 2011c, 2010b, 2009b. MacOsX 10.8.5, 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.
For quicker stem change entry on PM 2011, hyperscribe or manually input your music, then go back to the start where you want your stems flipped, click on the note, click "L" and use the right arrow on your comp. keyboard to go to the next note you want flipped. You don't have to manually change the value of each note. Oh, left arrow on keyboard will go back one note. Thanks everyone for your help, this forum rocks!