High Sierra is a version of the Mac OS that is installed by downloading from the App Store.
That said, unless you have a good, uninterrupted, internet connection with the App Store the install is likely to fail.
The complete download of High Sierra is in excess of 5GB, with the internet connection remaining stable throughout.
A possible solution is to download the entire thing without installing it, just saving to the hard drive ready for use.
For complete instructions on how to do this go here:
and read it carefully, a couple of times.
Let us know how you get on
Trying to install High Sierra on my mid-2010 iMac currently running 10.12.1 and having problems getting the whole thing to complete. After the agreement acknowledgement, I click continue and will get a message saying "In order to continue installing macOS High Seierra, you need to log in as administrator. Install macOS High Sierra is trying to add a new helper tool." I click on "Add helper", it gets down to as little as 4 minutes remaining and then it just stops. I've tried quitting and restarting it at least six time and will get down to varying amounts of time remaining and stops each time.
What can I do get this this install to complete. The last attempt got down to 27 minutes before stopping.