So that you don't mess up other permissions already on the file, use the flag +, such as via:
sudo chmod -R o+rw /htdocs/domain.com/public_html
sudo chown {replace_with_username} /htdocs/domain.com/public_html
sudo usermod -d /var/www/site john
chown root:root /var/www/site
sudo chown -R www-data:www-data /var/www/site
chmod 755 /var/www/site