It is defined in the output style selected. most author, year styles are alphabetical by first author surname. Numbered can be by appearance or alphabetically ordered. Edit>output styles> edit “the output style you are using”
The setting is under Bibliography, sort order. see attached image.
Then “save as” and rename as appropriate (recommend you don’t use the exact same name).
then make sure you change the output style to the newly named revised output style in the DOCUMENT on the endnote ribbon – not in Endnote program itself. The first time you may have to show all styles, to find the new one, as it may not appear in the dropdown yet.
