Copying an installation of drupal 8 to a subdomain

I have done countless backups and reinstalls of drupal 7 but I have hit a brickwall with drupal 8

I had done a backup via Softaculous of a non production site and also manual backup, but when trying to upgrade to 8.5.6 it killed it and I have not been able to successfully reinstall the backups. I am doing manual ftp, I have no knowledge of drush or any other method.

I have a smaller version of the now dead site and I tried to clone it via softaculous but the result is an incomplete installation. The core is missing and my host has not responded after several hours.

So the next step was to download the site and re upload and export/import the data base but I am completely messing this up and I have no idea how to fix it.

I have tried both by a Softaculous install and then import the database and a complete upload via ftp and then to import the database and all end in failure.

The database has a prefix from softaculous, do I delete it or what do I do?

Some step by step instructions on how to go about this would be very helpful.

Thanks for reading.

Moving to Post Installation as requested.

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