Once things have settled in with your Windows 10 install you might want to recover the space used by the Windows10.old folder.
To do this deleting previous Windows installations has now been incorporated into the Disk Cleanup Utility.
After launching Disk Cleanup click the 'Clean up system files' button.
You can now select to remove the previous install along with temporary install files and log files. In this instance I can regain a whopping 21GB of space!
Confirm the file deletion when prompted.
You'll be informed that rollback to the previous release will not longer be possible if the files are removed.
The files will be removed from your system