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Simply edit whichever fields need to be changed (or upload new files) and select the "Submit" button.
Copying widgets
To copy a widget (eg to create multiple widgets with similar fields), simply select the widget to copy in the drop down menu at the top, and then choose the "Copy Current Widget" item in the same menu. Change the title and then edit the fields as normal.
Deleting widgets
To delete a widget, log-in and select it like for editing above, but then press the "Delete" button at the top.