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.