Firmware revision number? Under "about" Because mines : 1028.3512.1337.0024
Where did you get that number from?
Which phone do you have? The latest Icon firmare on Denim is what gernerttl said, 02540.00019.15024.38022. Even without Denim, you should have had firmware that ended with 0047 if you own an Icon, I believe. Unless you didn't have the latest firmware even before Denim, I suppose.
The OS version won't change, it's the same as PfD if I recall correctly.
Make sure you have the Preview for Developers app installed and the checkbox in it checked. Then go and check for updates in Settings. It might sit there for a LOONG time looking for updates but evenetually you'll get it. If you don't, I don't know what to do. You sure you have an Icon?