As described in Bluebook rule 4.2, a shortened supra reference is used when an authority has already been fully cited. The following illustrates the format of a complete initial reference (footnote 43) and a subsequent supra reference back to the original (footnote 46).
As footnote references are added or removed, it can change the numbering of the original reference, which requires any supra reference to the original to be updated. Manually updating supra references is tedious and can lead to mistakes, so consider adding cross references to automate the updating process by following these steps.
To insert a cross reference within a footnote:
Place your cursor at the place in the footnote where you want the footnote number to appear. Next, select References > Cross-reference from the top toolbar.
When the cross-reference pop-up menu appears, change the Reference type to Footnote, and the Insert reference menu will automatically change to Footnote number. Leave the Insert as hyperlink box checked, select the number of the footnote the supra is referring back to from the list, then click Insert.
Updating all footnote cross-references in a document:
As you add or delete footnote references that change the numbering of your footnotes, you can update all cross-referenced footnotes at once by placing your cursor below the line (a.k.a. the area on the page where the footnotes appear) on any page of your document. Next, press Ctrl + A (PC) or Command + A (Mac) to select all text below the line, then press the F9 button (PC) or Function (fn) + F9 (Mac). A warning message will appear indicating the action can't be undone and asking if you want to proceed. Click yes, and all references will automatically update.