You will now see a list of folders containing includes and any includes stored in the root. Most of the includes you will need are stored in the root, these files are visible further down the page, beneath the folders. Either click the name of the include you wish to edit or click the Edit button on the right.
If you are struggling to find the correct include, on some newer sites the page preview tool will allow you to more easily find the file you need. To use this click 'Edit Pages' from the menu on the left. Locate the page containing the content you wish to edit and click the corresponding 'Preview' button on the right. You will now see a preview of the page with boxes surrounding various sections, click the edit button on the section you wish to edit. If you cannot see boxes surrounding different sections of the page your site's preview tool doesn't include this feature.