When Windows is upgraded from Windows 10 to Windows 11 a Windows.old is created.
The Windows.old allows the go back form Windows 11 to Windows 10.
The Windows.old by default is deleted after ten days.
It consumes drive space until it is automatically or manually deleted.
If you wanted to test drive Windows 11 for longer than ten days then the default period could be modified to sixty days.
During the sixty days the Windows.old will exist and enable the return to Windows 10.
If the computer user were happy with Windows 11 and decided to no longer return to Windows 10 they could manually delete Windows.old.
https://www.tenforums.com/tutorials/2066-how-delete-windows-old-windows-bt-folders-windows-10-a.html
To free up drive space third party software can be used to identify files for deletion:
Treesize, Windirstat, Wiztree, etc.
For freeing up drive space see these links:
https://www.tenforums.com/tutorials/83441-free-up-drive-space-windows-10-a.html
Once the Windows.old is no longer available it is still possible to move from Windows 11 to Windows 10 by a clean installation as licensing data is on Microsoft servers.
Important files need to be manually backed up to another disk drive or the cloud.
And drivers and applications need to be reinstalled.
https://www.tenforums.com/tutorials/1950-clean-install-windows-10-a.html
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