Ubuntu 15.10 will reach it's end of life on July 28, 2016

Ubuntu announced its 15.10 (Wily Werewolf) release almost 9 months
ago, on October 22, 2015. As a non-LTS release, 15.10 has a 9-month
month support cycle and, as such, the support period is now nearing
its end and Ubuntu 15.10 will reach end of life on Thursday, July
28th. At that time, Ubuntu Security Notices will no longer include
information or updated packages for Ubuntu 15.10.

The supported upgrade path from Ubuntu 15.10 is via Ubuntu 16.04.
Instructions and caveats for the upgrade may be found at:

https://help.ubuntu.com/community/XenialUpgrades

Ubuntu 16.04 continues to be actively supported with security updates
and select high-impact bug fixes. Announcements of security updates
for Ubuntu releases are sent to the ubuntu-security-announce mailing
list, information about which may be found at:

https://lists.ubuntu.com/mailman/listinfo/ubuntu-security-announce

Since its launch in October 2004 Ubuntu has become one of the most
highly regarded Linux distributions with millions of users in homes,
schools, businesses and governments around the world. Ubuntu is Open
Source software, costs nothing to download, and users are free to
customise or alter their software in order to meet their needs.