There are two ways to set up your selling order - Auto Assign or Manually Assign. This help article walks you through both options. 

This is your default setting for Selling Order. The system assigns seats closest to the stage first. The color gradient illustrates the order your seats will sell with orange colored seats being first priority seating and green seats being lower priority.

If you want to enable your own selling order, click selling order then select Manually Assign. If you do, the seat numbers will change to reflect the order they will be assigned. 

Note: You will have to assign a rank to each individual seat.