There are two ways you can move a buy order to a different status:
Use the "Status" dropdown on the Restock Page.
When viewing any of the buy order status queues, such as Awaiting Approval, you can click on the dropdown menu under the "Status" column and select the new status. The order will move as soon as you select the new status.
2. Use the Dropdown Menu at the bottom of an opened buy order and click "Update BuyOrder"
If you open an individual buy order and scroll to the bottom, you'll see the text: "Move from [current status] to" with a dropdown menu. Select the new status to which you want to move it, then click "Update BuyOrder".