There are two files that matter:
# /var/www/vhosts/default/htdocs/index.html # /var/www/vhosts/.skel/0/httpdocs/index.html
Also, if present:
# /var/www/vhosts/default/httpsdocs/index.html # /var/www/vhosts/.skel/0/httpsdocs/index.html
Windows Plesk files are in similar locations — C:\inetpub\vhosts\ instead of /var/www/vhosts
Edit index.html after backing up.