Go to "My Trips" in the Fly Delta App or on the website. Select your reservation, and if eligible, you will see an option to "Select Seats" for miles.
Upgrading your Delta flight with miles is a flexible process that can be done during booking or for an existing reservation, provided you didn't book a ticket. How to Upgrade with Miles delta upgrade with miles
If you can't find an option online, you may need to call a SkyMiles Representative . This is often required for international upgrades or partner-operated flights. Upgrade Eligibility & Limitations Miles Upgrades and More | Delta Air Lines Go to "My Trips" in the Fly Delta App or on the website