https://www.bleepingcomputer.com/news/microsoft/microsoft-says-windows-10-1809-users-can-dismiss-1903-update-alerts/

Microsoft says that Windows 10, version 1809 users who have recently started receiving update notifications to install the Windows 10, version 1903 can dismiss them and update at a later time.

"Updating remains in your control. To install the update, you must select one of the following options: 'Pick a Time', 'Restart Tonight,' or 'Restart Now,' says Microsoft in a recent Windows 10 message center announcement.