Pre-purchase look at the fare rules. The first letter is the coach fare class.

Post-purchase you'll need to call.

Note if this is domestic you can click on a seat in F on seatmap and it'll give you a cost to upgrade. Also pulling up the reservation at delta.com you can be given an upgrade amount in both miles or cash for any reservation including international. These are random, so if you see one you like take it. Theres no promise next time you pull up the reservation it'll be same amount, or even be there.