Simply navigate to 'Scheduled Emails' in the sidebar menu.

Then click on the gear icon in the top left corner of the scheduled email you want to modify.

After clicking the gear icon, a window will pop up on the right side of the screen (see below). From here you are able to add/remove recipients or modify the format and frequency. Once you have updated everything click apply.

Note: The only items you are unable to modify are the filters that were applied when the scheduled email was created. In order to modify those, you will need to create a new scheduled email.