回答“Is there a guide for changing batteries?”This battery is really simple to change.. The problem is that that battery may Crash and be really danger for technician. I suggest to cover with any piece of strong metal. Apple give a special protection to remove this battery.
回答“iPhone5c is locked on itunes”Explain better. Locked on Activation Lock or locked with some error ( 1015, 30xx etc ? )
回答“iPhone 5S shows low battery black screen even when it's not connected.”May be the problem is the lightning connector, shorted somewhere or oxided yet. Look inside it, try to clean with trieline, or also you cant try to brush slowly it. Let me know