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