Once you've located a page in the project that you want to upload or manage content for, go to the Child Pages section and then you'll see a list of files (content) currently displayed on the page. Just click the Edit button and the page will toggle to enable you to add/remove items and drag-and-drop to reorder them.
Click + Add an Item to add another file to the list. From there, you can choose a file on your device to upload. See the supported files list for content that you can upload.
Tip: you can upload multiple files in this step by holding down Shift on your keyboard while selecting files from your device.