The upgrade for Jenkins on teiJenkins was being kept back, and it turned out this was because Ubuntu 14.04 has Java 7 by default. I added a PPA for Java 8, updated the alternatives (sudo update-alternatives --config java) to point to the new one, and was then able to install Java 8. Following that, the Jenkins update went ahead. I elected to keep my existing config for Jenkins rather than overwrite. It needed a reboot for Apache to find Jenkins again.