Upgrade to High-Sierra failed

I am running iMac with Sierra installed. To upgrade to High-Sierra I used Net-Update (recommended in a email from Apple). After downloading the complete Sierra, an error occured and there was no way to boot anymore.

Now, using the Option Key I get the chance to select the 'OS X Base System' which tells me that:

"mac OS could not be installed on your computer, the path /System/Installation/Packages/OSinstall.mpkg appears to be missing or damaged"

--> this "OS X Base System (image) of High Sierra cannot get unmounted.

** I tried to get a bootable OS X stick from Apple, to solve the problem. Unfortunately they dont have this.

** I have a complete Backup, however, without a running OS X its of no use.

Hope for help. Thank you very much.