1. Go to the heading titled 'ARTICLE XXV'. Copy that paragraph's break (it uses auto-numbering, for the text, so you can't actually copy the text).
2. Go to the heading titled 'ARTICLE XXVI'. Paste the copied content immediately before that paragraph's break.
3. Delete the 'XXVI'
4. Click on the highlighted 'Numbering' dropdown on the Ribbon, then on 'Select Numbering Value' and set the value to XXVI.
Done.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]