You'll need a github user account. Make sure you're logged in to github.
At the lower-right of any page on this site, there's a link marked "edit_page". Clicking this should take you to an editing window on the github website.
The pages are written in Markdown syntax. Edit the text, and submit the form. (The submit button is marked "Propose file change")
Select "Create pull request"
If we've made you an admin for the website project, that's all you need to do - the edit should be made live within a few minutes. If not, then submitting the form creates a "Pull Request" that one of the admins should approve.