I would contact DELL and see if they can modify the order for you. They would be best suited to assist you with this. The OEM memory is specific to DELL and it's cheaper if you order it when ordering the machine as opposed to order it later, and it's sometimes not easy to find the correct DELL OEM memory. It's highly unlikely you will find it online as it's OEM only.
Is there any way to change my order?If not, can I purchase another 16GB ram and run it in dual channel?ITEM number for the RAM is 370-AGKL but I can't seem to find it online
Placed an R12 order couple days ago and realized I selected 16GB Single Channel 
DDR4 XMP at 3400MHz