Dear A UK User,
Good day! Thank you for posting your query in our community.
Per your description, you want to modify the drop-down list in the Excel workbook for connected Visio Cross-Functional Flow Chart (horizontal).
I reseached and tested on my side, however, Visio doesn't have a direct integration with Excel that allows you to modify drop-down lists in an Excel workbook from within a connected Visio Cross-Functional Flow Chart (horizontal).
Given this situation, we suggest you can raise this idea on Visio UserVoice at your convenience which it can let the related product team aware this limitation and improve it in the future. Your feedback helps us know which features are most important to you.
Your understanding will be highly appreciated.
Sincerely,
Jazlyn | Microsoft Community Moderator