Windows 10 will notify you when the April 2018 Update is available for your device. When the update is ready, you'll be asked to pick a time to install it. After it’s installed, your device will be running Windows 10, version 1803. To see which version of Windows 10 your device is currently running, select the Start button, then select Settings > System > About.
If you want to install the update now, select the Start button, then select Settings > Update & security > Windows Update. Select Check for updates. If updates are available, install them.