The rule is that if you arrive less than ten minutes before your flight is scheduled to depart, your reservation may be cancelled. To avoid this possibility you must personally check in at the counter or gate and have your name entered into the airline computer system. If the line is not too long you might be better off checking in at the counter. Even if you arrive on time you still run the risk of losing your seat if the flight is overbooked and you are one of the last to arrive. If you are flying within the United States, check-in time is usually one hour before departure.
International flights are usually two hours before departure.
If you change an entire ticket or the outbound flight it can be very costly. Usually a change fee and the difference in price between the old ticket and the new itinerary. If you are a queasy traveler you should consider motion sickness inhibitors such as pills or patches and take them prior to take-off. If you have a growling stomach, get something to eat or bring food onboard. Many flights no longer provide food service and even if they do, you may find that the selection is not to your liking.