In order to edit or delete some existing content, you simply have to find it.
A simple way is do what you would do when browsing the site - follow a menu item leading to it or type in it's address. Then you can click the Edit editing tab (if you are logged on) where you can edit or delete the content.
Alternatively, in the admin menu click on the Manage: Content menu item. This takes you to the Content 'View' which lists all the content on the site.
Our old site had nearly 2000 content pages and this site will soon grow, so make use of the 'Title', 'Content type', and 'Published status' filters. For example, set the content type filter to "Carousel hero" to list all the items in the front page carousel - which otherwise can not be found.
When you have found your page, you can view it by clicking on it's Title, edit or delete it from the Operations drop down menu on the right of the row. You can sort the list by clicking on the column title.
A third way to locate your page is to use the search bar in the Top Header row. With this you can search for phrases within in the body of the content.