When building your Purchase Order, you can edit your delivery method before you submit the order.
To do so when in your Purchase Order Quote:
At the bottom left of the screen, click the Previous button until you get to step 1, Delivery Options
Locate the Delivery Method drop-down
Click the box, and select your new preferred delivery method
Confirm details
Click Next to finish building your PO
If you have converted the Quote into a Purchase Order and it's been submitted to the branch (status will be In Progress) then you must contact your sales rep!