If you're thinking about changing your details, perhaps because you want to give your account to someone else, or perhaps someone is logging into your account and you don't know who it is, you'll first need to log in to your account, select Edit Account, then select Information and Password, and finally change it. 

This is actually quite common. Many people buy a ticket, check in, and then something unexpected happens that prevents them from making the trip. But don't worry, you can still change that. If you can't make your flight, you should contact reservations before your flight departs, check the value of your ticket, and see if you can keep it. You may be charged some penalties or fees for changes, but it all depends on the ticket. 

Unfortunately, no. After check-in, you won't be able to change anything on your ticket—including details, names, or reservations—because check-in is basically you confirming that you agree with everything on the ticket and that you're ready to board your flight. Consequently, if you have a reservation, you're good to go. However, if you really want to do something, you can contact reservations for help.