Delta Airlines Seat Upgrade offers several options for upgrading your seat to a more comfortable or premium class.

Buy an Upgrade: You can purchase an upgrade to Delta Comfort+ or Delta One directly through the airline's website, mobile app, or airport ticket counter.
Bid for Upgrade: You may have the option to place a bid for an upgrade through Delta's Upgrade Bid program. You will be notified before your flight if your bid has been accepted.
Medallion Status: If you are a Delta SkyMiles Medallion member, you may receive complimentary upgrades based on your status and availability.
SkyMiles Miles: You can use SkyMiles miles to upgrade your seat on select Delta-operated flights.

To know complete information about it read the full Blog.