Windows 10 will automatically download the October 2018 Update on your eligible device if you've turned on automatic updates in Windows Update settings. 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 1809. See some notes:
If you want to install the update now, select Start > Settings > Update & Security > Windows Update, and then select Check for updates. If updates are available, install them.