Each of your Categories, Groups, Headers or Super Headers has an S.No (Series Number) which determines its position within its grouping. To change this, go to Settings > Manage Categories, or Manage Group, Manage Header or Manage Super Headers and click the up down arrow icon:
to the right of the screen to switch to Reorder Mode. Then simply drag and drop the row to the place you want and click save. Now all your dashboards and Excel reports will follow this new ordering.
You will only be able to reorder Categories within their current Group, Groups within their current Header etc. If you wish to move a Category to a different Group, select the pencil tool on the right.