Windows 10 Update Cleanup

Windows 10 feature updates now come two times a year, in March and September, 

i.e. 1703 is March 2017, 1709 is September 2017 etc. 

Those updates are in fact full installs in a separate folder, and then all files and apps are copied from previous version. That takes a long time, and previous version takes a lot of space (20GB or more). 

The old version is saved in "C:\Windows.old" folder, to be available for "undo" if needed.
To reclaim space from previous version:

How to delete the Windows.old folder from Windows 10 - CNET
'proper way to delete the Windows.old folder:

1. Click in Windows' search field, type Cleanup, then click Disk Cleanup.
2. Click the "Clean up system files" button.
3. Wait a bit while Windows scans for files, then scroll down the list until you see "Previous Windows installation(s)."'