How to downgrade OS7 version on a Mac?

Hi, trying to downgrade my Sprint 9930 from 7.1 Bundle 1737 to an older version to see if reception might improve. I am a total Mac user, and have backed up to Desktop Manager a few times with this phone. I'm trying to select "Show Other Versions" per this screenshot below, but the button for that option is greyed out, even though the dialogue box says "You can return to a previous version of BB device software" . Any help / advice on how to simply do this - and which version might be best while i'm at it - much appreciated.

Thanks,

Mike