Windows 10 build 21390.1000

Windows 10 build 21390.1000 is an update to build 21390.1 that was released to the Dev Channel on 7 June 2021 and, according to Microsoft, 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.