Windows 10 version 1903, also known as the May 2019 Update, will reach end of support next month on December 8. As usual, new Windows 10 updates released in the first half of year only get 18 months of support, and the May 2019 Update will no longer receive security updates after December.
If you’re still running the version 1903 of Windows 10 on your PC, Microsoft now recommends to install the version 1909 (aka the November 2019 Update) to continue receiving security and quality updates on your PC. The company will also push this update automatically to PCs still running the version 1903 to keep these devices protected in the following months (via Neowin)
According to AdDuplex’s latest survey, 22% of Windows 10 PCs were still on version 1903 last month, compared to 32.4% for the version 1909. Both versions actually share a common core, and the update process from version 1903 to 1909 should take less than 10 minutes. You can visit Windows Update now to start the update process, though it's recommended to back up your files before proceeding.
Windows 10 version 1909 will get 30 months of support, like all Windows 10 versions released in the Fall. That's also the case for the October 2020 Update (20H2) which the company started rolling out last month.