You can return ammo that is in your Queue that hasn't yet been allocated to your Inventory. Just find the Crate you want to return ammo from, go to your Queue and click the "Return" button next to the caliber you want to get a credit for:
You'll see a screen with the Return information as a final confirmation:
Just click the yellow button marked "Return Rounds" at the bottom of the confirmation page to complete the transaction.
Please Note: Unlike active inventory, you don't pay a 5% fee if you are returning ammo from your queue because the ammo hasn't yet been allocated to you - you are essentially giving up your place in line for that ammo. Likewise, when you return ammo from your queue you are just getting an account credit for whatever price you paid - even if the price per round has changed since you were first put on the queue.