Windows 10 build 21390.1000

Windows 10 build 21390.1000 is the first update of build 21390.1. This build was released on 7 June 2021 to Windows Insiders in the Dev Channel. According to Microsoft, this is the first in a series of updates in order to test the process of releasing multiple cumulative updates on top of each other. However, several enthusiasts pointed out that Microsoft is releasing cumulative updates because the developers were at the time currently working on merging the Sun Valley work in preparation for the Microsoft event on 24 June.