It's been two years and three major updates since Windows 10 launched. Despite this there are a number of users still on the build 1507 launch version, having disabled updates straight away. However, that small percentage may want to upgrade soon, or they will become insecure and miss features.
“After May 9, 2017, Windows 10 devices running version 1507 will no longer receive security and quality updates. Microsoft recommends updating devices to the latest version of Windows 10 by visiting the Software Download site and selecting Update now to manually update your device. For additional info, check out the Windows lifecycle fact sheet.”
The move was expected and a vital part of Microsoft's new policy. Windows 10 is supposed to be the last OS, letting the team focus on making one system better. However, that would be jeopardized if Microsoft had to maintain Windows across tons of different versions. Instead, the company will support one major edition.
Windows 10 Version 1511
After the Windows 10 launch version, the next build is 1511, so users will want to upgrade to that before May. However, it's likely that version won't last long either. On the current schedule, 1511 will lose support sometime in October.
Of course, LTSB customers will not be affected by this change. Windows 10 Enterprise customers on the special plan have guaranteed support until a later date. Other users won't be forced to upgrade in any way, but will gradually become more open to attacks as new methods aren't patched.
You can check which version of Windows you're on by typing winver in the search bar. You should then be able to update via the manual tool.