If you already have 15.04 Beta 1 installed, just run the following terminal (Ctrl + Alt + t) command:
sudo apt-get update && sudo apt-get dist-upgrade
and it should/will update to the latest version!.
If you want to upgrade from 14.10; I would wait for the 15.04 Final to come out and use the software updater tool which depends on your software sources settings if you have it set to "Never" where it states "Notify me of a new Ubuntu version" in Software & Updates > Updates, you need to change that to "Any new version"!.
If you want to skip the above, use the "Something else method" described here: