The Windows 10 update saves files from your previous installation so that you can revert back to it if you need to. Deleting those files can get you back up to 20 GB of disk space.
If you updated to Windows 10, you may notice a fair bit of disk space missing. Windows saves files for previous versions of Windows so that you can uninstall Windows 10 should the need arise. Those files can eat up gigabytes of disk space. If you have a large drive, that may not be a big deal. But if you're running low and you're happy with your Windows 10 installation, reclaiming that space is easy. Just open up the Disk Cleanup utility, click Clean Up System Files, and select Previous Windows Installation(s) to be deleted, along with whatever else you don't need. I was able to reclaim 22 GB, not a small amount on my relatively little SSD.