If you open your reservation on the website, if you scroll down, you may see an offer from any time after purchase. Similarly, on the app, if you load up a reservation (usually after a few seconds), there may be a bar on the top that gives you a link to upgrade. This can also happen any time after purchase of the ticket. You should note that the price of upgrades does vary and can change literally minute to minute. If it is an upgrade you really want, I'd recommend checking daily to see what the offer of the day is. If you see one you like, go for it.