I have installed Powertop from the Ubuntu Software Center and just realized that a new version is available. However this new version is not yet available in the Ubuntu Software Center. How do I then upgrade to the new version?
Powertop 2.1, based on the source from the Ubuntu 12.10 version, is available in my bleeding-edge PPA.
To add the PPA and install Powertop 2.1, please open a terminal with Ctrl+Alt+T and type/paste these lines (one at a time):
Traditionally major updates to applications are only backported to Ubuntu releases when they contain security fixes or unignorable benefits to users. You might want to track down powertop's Ubuntu maintainer to see what they feel.
Otherwise, you're left to get this from another source. In this case I think you should be fairly safe to look at the available builds here:
... And just snatch Quantal's version. It's dirty and you won't get upgrades for it but I don't think anybody is going to discover a remote exploit in Powertop so I think you're probably safe.
Other options including finding a PPA that hosts it or even downloading the source and building it yourself.