This is a good guideline to follow before sending your device for service - https://support.apple.com/en-my/HT201557
Here are some important points to note.
1) Make sure all data is backed-up (the Service Center do not provide data back-up service, all data will be erased once the unit is surrendered)
2) Disable Find My iPhone (FMiP) feature from the unit.
3) The program will replace only the battery.
6) No loaner/temporary unit will be provided.
7) Accessories such as screen protector/tampered glass is not covered and consider loss once the unit is surrendered.