If I understand correctly ?
A newer version of macOS was attempted to be installed and it did not complete successfully ?
Now you what to revert the operating system to a previous working version of macOS ?
Am I right so far ??
Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the internet.
It will present options >> Disk Utilities >> View >> View ALL attached Drives.
Choose the Upper Most Drive ( not the volumes indented and list below ).
The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above.
Once that is done >> backup out of Disk Utilities and choose install macOS.
Follow the prompts and it may automatically reboot several time.
Upon a final reboot - Setup Assist will present .