ref Version 10.0.14393 Build 14393
Am not entirely sure what Microsoft is thinking but now have been through the process to update two Win 10 32GB tabs to Windows 10 Anniversary
You need over 15GB free to get past the first check but then started getting an error 80070070 "insufficient free space". This continued until I had freed up 18.2GB on disk C. No option to use external disk as with Win 10.
This is not for the faint of heart as in the end I was moving whole program directories and "gigantic" files to the D drive (sdCard).
However it can be done. Whether the new build is any good is yet to be seen.