My ubuntu is 12.04, I tried to follow the instructions in: http://www.ubuntu.com/download/desktop/upgrade, but the upgrade button does not appear.
Have you tried going into settings and changing the value for when to look for distro upgrades? by default I think it's set you only look for long term updates, you need to change this to "For Any Version"
Update Manager > Setting > change "Notify Me Of A New Ubuntu Version" to "For All Versions"
How this will help you?