To add TSA PreCheck to Delta, you can either add it to a new reservation or to an existing reservation.
To add it to a new reservation, log in to your Delta SkyMiles account, find the reservation you want to add TSA PreCheck to, and under "Traveler Info," click "Edit." In the "Known Traveler Number" field, enter your TSA PreCheck Known Traveler Number (KTN). Click "Save."
To add it to an existing reservation, go to the Delta website or app and sign in to your account. Click on "My Trips." Find the reservation you want to add TSA PreCheck to and click on the "Edit" button. In the "Known Traveler Number" field, enter your TSA PreCheck KTN. Click on the "Save" button.
Your TSA PreCheck number will be added to your Delta profile and will be applied to all future reservations that you make.