Most aspects of your website can be edited using the page or the post editors. This is how you can delete, edit or create new content for your site.
There is one area where it is not so obvious how you can modify the contents.
If your site has a sidebar then this is managed from the Appearance Menu. View this article on Widgets to understand how your sidebar is managed, and how you can change the contents presented in your sidebar.
Take care here, HTML and other code can be presented in the sidebar, so it is not like editing a page or post in the Visual editor.