Help:Introduction to referencing with VisualEditor/3
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To edit an existing reference, click on the [number] where it appears in the text. You will see either a 
"Reference icon", or the icon (and name) for the template that was used to create this reference. Clicking on "⧼visualeditor-contextitemwidget-label-secondary⧽" button opens a "template mini-editor" dialog where you can edit the information for that reference.
Inside the template mini-editor, you change the information in fields by typing in the boxes.
To add a new piece of information (for example, the publication date), click on "⧼visualeditor-dialog-transclusion-add-param⧽", which is at the very bottom of the mini-editor.
When you're done editing the reference, click "⧼visualeditor-dialog-action-apply⧽". Remember, you'll still need to then save your changes!