I couldn't find it for the life of me on Tuesday, but today for my return trip it was very prominently displayed in an orange bar in the middle of my flight details screen. I am SURE it was not there on Tuesday and I just overlooked it; I think at some point this week the app got an update, or perhaps some flights just aren't showing up for some reason.

Don't feel dunce-y; when it does show up properly you won't be able to miss it:

Sorry for the crappy photo...I'm at 35k feet right now and couldn't think of a better way to do it.

Edit: And despite what the app says, I did get my upgrade right before takeoff!