Sometime only the owner of a file has editing rights (and sometimes no one has write permission -- it depends on how the files' permission is set). Please post the output of:
ls -l /var/www/html
This command will show you the permission of all the files and directories in /var/www/html.
Do you have root access to the system? The root user can edit any file and manipulate its permissions. However, it's not good to do day to day work as root, so you should set the permission of the files you want to edit such that your user account can modify them as needed.