My sisters iPhone X screen randomly presses on stuff or becomes unresponsive

my sister left her phone outside in the sun one day when she went swimming and the phone overheated. Ever since then she has had an issue where sometimes when she is trying to message someone the phone will start pressing on letters itself or she'll try to go to a different page on her home screen and it will be unresponsive. We have warranty if we have to use it but i was wondering if this is something that we can fix ourselves or if we had to get an entirely new phone. Thank you.