Thanks. I hadn't noticed that.

Running dpkg -i * as root in the download directory appears to work and show all of the right messages but on starting Opera it still shows version 12.16 on the help menu. I tried repeating the installation:

(Reading database ... 343088 files and directories currently installed.)
Preparing to unpack opera-beta_26.0.1656.17_amd64.deb ...
Unpacking opera-beta (26.0.1656.17) over (26.0.1656.17) ...
Setting up opera-beta (26.0.1656.17) ...
Processing triggers for hicolor-icon-theme (0.13-1) ...
Processing triggers for gnome-menus (3.10.1-0ubuntu2) ...
Processing triggers for desktop-file-utils (0.22-1ubuntu1) ...
Processing triggers for mime-support (3.54ubuntu1) ...

It says that it's installing over 26.0.1656.17 but the installed version stays at 12.16. opera --version reports Opera 12.16 Build 1860 for Linux x86_64.

What do I try next?