You need access to the web server hosting the website. Such access can usually be accomplished either via access to a control panel opened with your web browser by going to a special web page of the hosting service, or by using FTP software to connect directly from your computer to the web server. In either case, you need a username and password. And, obviously, you would need to either be the person of developed the web page, created the account with the hosting service, and published the web page to the world wide web, or you would need to be authorized as an administrator by the person who has already created and published the web page.