You change the tabbing order by selecting the objects and changing their positions in the Tab Order palette list.
Drag the selected object up or down the list and place it at the required location. A black handle marks your current position within the list before you place the object.
In the Tab Order palette list, click the selected object to make the number listed beside the object name editable. Then, type the new number indicating the new position of the object in the tabbing order and press Enter.
Select Copy from the Tab Order palette menu and, in the list, select the object above which to place the object you are moving, and then select Paste from the menu.