zdnet[.]com/article/windows-11-give-yourself-more-time-to-roll-back-the-upgrade/
docs.microsoft[.]com/en-us/windows-hardware/manufacture/desktop/dism-uninstallos-command-line-options?view=windows-11
When a Windows version upgrade requires a reinstall -- Not a cumulative update -- your old Windows files are moved into a folder called Windows.old, and you can choose to Roll Back that update to get the old version back as it was. To be clear, at least IMHO, you're better off to have an image backup you can restore -- it's faster, since you're simply writing data rather than deleting &/or moving files around, and more reliable, since the software code & its logic are simpler, meaning less to go wrong. But not everyone agrees that backing up is worth the time and effort.
In that case, rolling back to the prior version is your only option, other than reinstalling that earlier version of Windows, and you've only got 10 days to do it, before Windows deletes all those old, roll back files. Ed Bott [ZDNet] dug up the way Microsoft lets you extend that period from 10 days to up to 60 days. Do bear in mind that jumping back 10 or 20 or 60 days means that you'll need to apply whatever updates were released since then, and you should also make sure that you safely store any downloads, emails etc. that you want to keep, since that stuff could get wiped out along with that newer version of Windows.
You change Windows Roll Back settings with the Windows DISM tool, using the Command Prompt -- to get the Command Prompt window use search, or it may be in the menu when you right click the Start Button, or find it in the Start Menu under System [right click the icon or shortcut -> more -> Run as admin.]. In WIn11 you can also find it the Control Panel -> Windows Tools folder. The command is:
DISM /Online /Set-OSUninstallWindow /Value:<days>
There are several other related functions you can use on the linked Microsoft page, including deleting those roll back files to reclaim the disk space, though Disk CLeanup also works.