Have you tried logging into the server e.g. cpanel file manager or whatever server admin panel you have, then delete the files from there? Also if you have a db backup, you could try reinstalling that too, then trying again ensuring you upload all files to the correct place (I use FileZilla as it’s very reliable).
If you don’t have a DB backup (first qstn is why? you should always have regular backups!) maybe take a look in the DB to see if there are any updraft entries you can delete – but that is more tricky and you can damage the DB so be careful!