By default, the order of the specification is based on the order in which you click the "S" button within an estimate.

Like many areas of Buildxact the order can be changed by grabbing the dots on the left and dragging to re-order

If you want to reset the order back to that of the estimate you can select the 'Match costings order' button shown below.

Note: categories which have been manually added will not be affected by resetting the order