Ubuntu releases come in two flavours, bog standard and Long Term Support (LTS). The latter is supported for five years while standard releases are supported for just nine months. Today, Ubuntu 15.10, a standard release, has reached its end of life, leaving just three releases in a supported state; they are 12.04 LTS, 14.04 LTS, and 16.04 LTS.
Many Ubuntu releases come and go these days without too much difference between them so it might be difficult for you to find which release you're running, a good way to find out which version of Ubuntu you are running is by running the following command in the terminal:
If you are indeed running Ubuntu 15.10 or 14.04 LTS, you are able to upgrade to the latest LTS. Open the update manager, if your settings are configured correctly a prompt will appear letting you know that an upgrade to Ubuntu 16.04(.1) is available. Ubuntu 15.10 users are advised to upgrade right away to continue receiving security updates.
Last week, Canonical released Ubuntu 16.04.1, the first point upgrade or service pack for the LTS release. Users on the previous LTS, 14.04 were not offered Ubuntu 16.04 from the update manager when it launched. This gave Canonical time to squash bugs found in post-release. With the availability of 16.04.1, 14.04 users were given the option to upgrade.
Source: OMG Ubuntu