The app to install High Sierra Doesn't Work, and so now what?

I'm updating my laptop from Sierra to High Sierra, downloaded the software & the install app. Closed all other apps, and the installer, is just about to restart and begin installing it, and an error window pops up saying something went wrong with the app and to "try again". Great, so I try again, and again, and then again, and still nothing! I go back to the App Store thinking I could download another install app, but when I do a search for High Sierra, it of course shows nothing! What kind of insanity is this, that in Apple's own App Store, where I just downloaded the software an hour ago, now gives me no help whatsoever, and the software I just downloaded can't even be found in a search? And of course, there's no suggestions of how to remedy this impasse, beyond "TRYING AGAIN"???