You can often do the following directly with the airline, right on their website!
View your full itinerary
Select your seats
Purchase add-ons (upgraded seats, meals, etc.)
Add a frequent flyer number
Add assistance (wheelchair, etc.)
Check-in and print your boarding pass
If the airline you're flying with allows you to manage your booking online, you can get to their site right from your trip in the app. Here's how:
Open your upcoming trip and scroll down to the bottom where it says "What can we help you with?" or "Get Help" and tap on "Check In & Manage Booking:"
Next, tap "Copy & Go" to copy the airline confirmation code and head straight to the airline's website. You'll need to paste that copied confirmation code and enter your name to access the booking.
Good to know
Some airlines will not allow you to manage your booking online because the booking was not made directly through them. Relax! Your booking is still confirmed and you can check-in at the airport on the day of departure.
If you have several last names, you may be required to enter them without any spaces in between.