Win10 failed to upgrade to Win11 via built-in updates. After restarting, it remained on Win10.

When upgrading a Windows 10 Home PC (C drive at 89GB, with over 10GB free space) to Windows 11 via the official update tool, the system reboots after reaching 100% progress but reverts back to the Windows 10 interface. During the upgrade, a prompt requested at least 10GB of external storage space; a 120GB USB drive was used to complete the operation. All data on the C drive must be preserved. Seeking a non-formatting method to repair the upgrade.