You cant publish single sheet to update the dashboard which is already in the server because it simply overwrite it.
I fear there is no option to update the dashboard
Hope this clears confusion
If it helps kindly mark this helpful/correct so that it will help others
Publishing a workbook is all or nothing as far as I know.
You might be able to edit the workbook on the server to introduce into the published workbook the changes you've made in Desktop. (I'm not a big fan of editing on the server, but this might be a way to do what you need.)