After Windows 10 installs, it leaves behind a folder called Windows.old. That folder contains your old operating system. It's left on your PC in case you want to revert back to Windows 7 or Windows 8 after installing Windows 10. You'll find the folder underneath your new Windows folder for Windows 10.
If you don't want to revert to your old operating system, though, it's just wasted space, and lots of it. So you can delete it without causing problems on your system. You can't delete it like any folder, though. Instead, you'll have to use Windows 10's Disk Cleanup tool.
To run it, type Disk Cleanup in the search bar, and then click the Disk Cleanup program entry that appears. Click "Clean up system files," and the tool examines your system for files to clean. After a short while it shows you a list of all the files it can clean for you.
Scroll down to "Previous Windows installation(s) and you'll see how much space your Windows.old folder uses. In my case it's 27.3 GB. Check the box next to it if you want to delete it, and then click OK. If you do that, of course, you won't be able to revert to your old operating system. You have only 30 days from the date of your Windows 10 installation to revert to it. It's expected that Windows itself will delete the folder after 30 days.
There's more you can clean up as well. Check the box next to "Temporary Windows Installation Files" to delete installation files you no longer need. In my case, that's 3.77 GB worth. So I was able to clean out more than 31 GB of files with a few clicks.