Many thanks, Jens. I now have 10.05 up and running.
For the benefit of others reading this correspondence, let me be more explicit about what I did. First, I did NOT find Code::Blocks 10.05 in the Ubuntu repositories, only 8.10. (I had looked there for 10.05 before posting my question.) So I added Jens's repository by editing the sources.list file in the /etc/apt directory to add the line
deb http://apt.jenslody.de/stable stable release
This file was protected from writing, so I first had to use a terminal to do
sudo chmod a+rw sources.list
and then use gedit to add the line.
Then I restarted the Synaptic Package Manager and looked for codeblocks. There was 10.05! When I clicked one of the old 8.10 files to update, I was sternly warned that the software could not be authenticated and that it might be produced by a malicious individual. Never having found Jens malicious, I decided to go ahead, and all seems well.
The icon on the desktop was not changed, but when I clicked it, 10.05 came up, not 8.10.