Most of the Haskell website is a wiki, meaning that content can be edited by everyone. However you have to create an account and log in if you want to change the site. This is to protect the wiki against spam. Don't worry: account creation is quick and easy.
On the bottom of each wiki page you will find links for creating an account, logging in, editing the current page, and so on. Help on editing is available, including information on editor support.