Hi jaxandr,
As is suggested by Ashidacchi, we may take the following steps to change the endnote number format:
1.Display the References tab of the ribbon.
2.Click the small icon at the lower-right corner of the Footnotes group. Word displays the Footnote and Endnote dialog box.
3. Select the Endnote radio button.
4.Using the Number Format drop-down list, select the numbering format you want to use:

5.Click OK to close the Footnote and Endnote dialog box.
For more information about how to add and customize footnote and endnote, you may refer to the article below:
https://support.office.com/en-us/article/Add-footnotes-and-endnotes-in-Word-61f3fb1a-4717-414c-9a8f-015a5f3ff4cb
Hope you can find this helpful.
Best regards,
Yuki Sun
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