You can edit or delete files or folders in your repository or repositories of groups you are an organizer in. Click the pencil icon to the right of the file or folder you're interested in.
From there, you can edit a file's name or description or you can upload a replacement file. If replacing the file, if the new file you upload shares the same file name (say, image.jpg) as the one you're replacing, the link to that file will not change. This means that you can replace an image's content in an email you've already been sent by uploading a new image with the same name, or update a CSS file used in your wrappers without having to update the wrapper code.
Click "Save File" when you're done. You can also delete the file by clicking the "Delete File" link and confirming the deletion.
Similarly, you can edit a folder's name or delete the folder in the same way. Deleting a folder automatically deletes all of its contents, so be careful!