Ya karmic will have it in the repo but its not hard to install. just get and extract the package from here
Ampache :: Developer Blog » 3.5.1 Released
In ubuntu: copy the extracted folder to /var/www/ampache-3.5.1
then point your browser to
http://domain.com/ampache-3.5.1 to start the install.
I would create a new database for the new version of ampache. You can use the same one but I am not sure of any problems that might occur with the old database.
You will also need to generate the config file and put it in
/vaw/www/ampache-3.5.1/config
Its all in the install script though.
This is probably not the most secure way of doing this. I just set it up quickly to get it working So I could work on the app.