How to delete old Windows install files and free up drive space

[Windows Central]

How do you get rid of old Windows installations?
When you update your PC from an existing Windows installation, the old files remain on your hard drive. They are saved there instead of being overwritten because something might go wrong during the update installation, or because you might want to roll your PC back to a previous installation if you aren't happy with the new stuff.
If, however, the update to Windows 10 goes smoothly, and you're sure you don't want to return to a previous version of Windows, you can delete those old installation files to free up space on your hard drive. This is especially important for anyone using a device with a smaller solid-state drive (SSD).

Full story from the WindowsCentral blog...