Existing services and categories can be re-arranged as needed on the 'Services' page.
Setup > Services
When you hover your mouse over the blank space in the action column, the mouse cursor will change to a multi-directional arrow allowing you to click and drag the service or category into a new position. Once in the position, simply release the mouse click to drop into place.
Please note: Service options and sub-options can not be moved independently, you must select the service or a category.