As of today, Ubuntu 15.10 (and its official flavours like Ubuntu MATE) reach end of life. There won’t be any more updates or security fixes for this release.
Still using 15.10?
You can check your current version by opening System Monitor and looking under the System tab.
If you’d like to upgrade to 16.04, which has a support period of 3 years, open the Software Updater and follow the upgrade prompts.
Use an AMD Graphics Card?
The proprietary driver is not compatible in 16.04. Instead, only the open source driver is available.