With my Vi10 Plus I found that I had an issue caused by lack of space — upgrading to 1607 would fail because it needed 20gb of space, which the 32gb emmc didn’t have.
To get around this, use the Media Creation Tool to make an installer USB for 1607 and then use the USB to upgrade.
After that, if you’ve got no data to lose, do a “Reset my PC” from Settings, and you’ll be able to recover a bit more space compared to a “clean” Chuwi install.