Can I upgrade my Delta seat after purchase?
Yes, you can upgrade your seat with Delta even after purchasing your flight ticket, but the options available will depend on your fare class and the availability of upgrades. Delta offers several ways to request an upgrade after purchase, including using the earned miles, paying with cash, or complimentary upgrade certificates if you're a SkyMiles Medallion member. The most common way to upgrade can be through the Delta Seat Upgrade program, where passengers can use miles to place a bid for an upgrade to a higher class of service. If you are not in the elite program, you can still request an upgrade, but they will be called complimentary. Also, you can pay for an upgrade at the time of booking or after booking through Delta's official website or get help from the customer service team.
Read more: https://www.airlinesgrouptravel.com/blog/how-to-upgrade-on-delta