Xubuntu core is a slimmed down version of Xubuntu that doesn’t come with all the additional features of a full and modern desktop. We essentially only ship Xfce and the basic look and feel of Xubuntu, so there will be no office suite, media players, et cetera.
See below for the latest, learn more about general testing and QA here: https://lists.ubuntu.com/archives/xubuntu-devel/2015-April/010721.html
If you follow the link in the email linked above, you'll land on a page that has all the tests. You want to scroll to the "Product (Upgrade)" section, and at the bottom of that are the two for Xubuntu, 64-bit:
Depending on which you want, follow the link, then you then click "Upgrade" for just doing an upgrade with the update manager (read the instructions) or "Upgrade (image)" if you want to do it by downloading an ISO and running the upgrade from there (again, read the instructions for exact directions).
(from Xubuntu Team Wiki)
"Xubuntu is a community developed, Ubuntu-based Linux operating system.
Xubuntu is an elegant and easy-to-use operating system. Xubuntu comes with Xfce, which is a stable, light and configurable desktop environment.
Xubuntu is perfect for those who want the most out of their desktops, laptops and netbooks with a modern look and enough features for efficient, daily usage. It works well on older hardware too."