Most likely, the file ownership and permissions only allow the root user to modify/edit that file.
In order to modify the file you need to login into the server via SSH (if it's at a remote location) or at the terminal and execute the command,
sudo vi /var/www/index.html
This will prompt you for your user login password. It should open the file index.html in a text editor vi with read and write access.
You need to check for the commands sudo and editor vi.