If you're still on the Windows 10 April 2018 Update, or version 1803, it's time to upgrade your PC. As of today, Windows 10 version 1803 is no longer supported for Home or Pro SKUs. If you're on an Enterprise or Education SKU, you've still got another year.
Beginning last July, Microsoft started automatically upgrading those on version 1803 to version 1903. These days, the company doesn't force upgrades like it used to. Now, you'll get an option to update to a newer version, and the only way that Windows will do it automatically is if the version that you're on is nearing the end of support.
As it stands right now, around 13.6% of Windows 10 users are still on version 1803, according to the latest report from AdDuplex. Naturally, that includes those on Enterprise and Education SKUs, which again, are still supported. Still, that's not a small number, and those users should expect to be moved to a newer version sooner, rather than later.
The last update for Windows 10 version 1803 still arrived today with the Patch Tuesday updates, so you're not in an unsecure state just yet. You'll want to upgrade your PC before you are though.