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Check out the guide Free up Gigabytes of disk space after Windows 10 Creators Update installation for additional information on the process. It is recommended to run this process after the upgrade to a new version of Windows once you know that you won't need to roll back anymore. Accept that, and continue with the deletion of the data. Windows may notify you at this point that deleting old Windows installation date will prevent you from going back to the previous version. Note the size of them, and check any other item there that you may want to delete.
Locate "temporary Windows installation files" and "previous Windows installation files" there and check the entries. Tap on the Windows-key, type Disk Cleanup, hold down the Shift-key and the Ctrl-key, and hit the Enter-key. You may delete the folders using Disk Cleanup, a built-in tool of the Windows operating system. This allows you to restore the state before the deletion of the folders, and roll back if the need arises. Note: You can create a system drive backup just in case, and delete the folders once you have done so. This means, that you should not delete the folders if you a) plan to go back, or b) don't know yet if you need to go back to the previous version. The folders are not use when you run the new version of Windows on the computer, but are required when you decide to roll back the upgrade and return the system to the previous version.