Chrissy has an iPhone 6, and she had to replace the screen twice. She's also suffering from overheating with her car navigation, and her calendar keeps erasing. Leo says it may be a good idea to reset your phone to factory defaults and starting from scratch. It'll erase the storage memory and reinstall the iOS. But back it up first. Then use it for a few days before you restore your backup. As for overheating, that may be your case. Try doing the car Nav without the case on and see if it overheats. If not, then it's time to get a new case.
Mike got an iPhone from his daughter and wants to know how to use it. Leo says that the first thing he needs to do is remove it from Find my iPhone (using her iCloud password), then wipe it. Once that's done, he can reset the iPhone and insert a new SIM. Here's how - https://support.apple.com/en-us/HT201252