If you want to install Windows 10 Version 21H2 using Windows Update, you will need to press the Download and install link on the Windows Update settings page. This is because initially 21H2 has been released as an optional update. The update will be an enablement package which is relatively small if you are currently on Windows 10 version 2004 or 20H2. Hence, the update should not take more than a few minutes.
Download Windows 10 v21H2
You may use the links provided below to download the ISO image for Windows 10 v21H2:
Download Windows 10 Version 21H2 ISO 64-bit English (5.5 GiB, 152,175 hits)
Download Windows 10 Version 21H2 ISO 32-bit English (3.9 GiB, 28,060 hits)