Microsoft released the first preview build of Windows 10 21H2 this week, but it is not being offered to everyone at this time. Microsoft is only making it available via Windows Update to Insiders who were moved to the ‘Release’ channel for not meeting Windows 11’s hardware requirements. The Windows sleuth Albacore has shared a method that allows users running Windows 10 2004, Windows 10 20H2, and Windows 21H1 to upgrade their device to the new feature update quickly.
Source: https://www.bleepingcomputer.com/news/microsoft/cant-download-windows-10-21h2-heres-how-to-get-it/