$ wget -O - http://deb.opera.com/archive.key | sudo apt-key add -
and I could upgrade to the new version.
If you don't have Opera repository in software sources add it:
I have installed on Ubuntu Jaunty but the repo should work on any version of Ubuntu.deb http://deb.opera.com/opera/ stable non-free
Personally I am dissatisfied with Opera since it does not render Devanagari fonts properly e.g. see how it renders this word स्पष्ट The correct rendering is