Option is only available at check-in time (and are traveling alone) --

"And starting June 15, 2017, when traveling alone, Medallion Members who are not opted in to receive automatic Delta Comfort+ Upgrades will now have the ability to request a Delta Comfort+ upgrade to any available seat or only a window/aisle seat when checking in for a flight at delta.com or at an airport kiosk."

https://www.delta.com/content/www/en...plus-faqs.html