If you were quick off the mark on 25 September and downloaded the first, small (19.9 MB) installer for the High Sierra upgrade, you might want to start again.
Although Apple released this two-step installer at first, it was later replaced by a 5.17 GB monolithic installer app, which works much like previous installers. The two-step installer was a small app which then downloaded the rest of the installation files, as described in my account of upgrading to High Sierra posted last night.
The immediate problem with the first version of the installer is that it is incomplete, and cannot be used to make a bootable installer on a USB drive, for example. I have no idea whether the version of High Sierra which it installs is the same as that in the second installer, or whether they are both functionally the same.
I have been unable to find any official account of what happened, but this appears very much like human error. Apple, you had one job on 25 September…