I remember frantically checking the clock before my trip to Chicago last year when a family emergency popped up. I wasn't sure if I’d already missed the cutoff! Through that experience, I learned that the best way to handle this is to act before the flight actually departs.
Technically, you can cancel right up until the boarding doors close, but I wouldn't recommend waiting that long. I used the United app to modify my plans about three hours before takeoff, and since I didn't "no-show," I kept my fare as a travel credit. It was such a relief!
Would you like me to check the specific "no-show" rules for your ticket type just to be safe?