You will first need to be logged in via the user page. For example, the following link would be used on this site: http://designsystem.wwu.edu/user
To add files on the website you will need to navigate to a different area of the website itself:
From here you can view, add, replace, or delete content.
To add content, you can click on the "Add Content" button or navigate to Content/Media/Add Media/Document
(or Image if you need to add a new image – but typically this is done on each page individually)
To add a new Document
Click "Choose File", find the file on your computer and select it, and then click "Save":