During a return process, the reset of your phone is important. Without this process applied, it is quite complicated to carry on the return process once the phone is received.
Here's an explanation of the reset process below:
First thing first! We advise you to save all of your datas before starting the reset.
Go to the phone settings then "General":
Go to "Transfer or Reset iPhone". You can find it at the bottom of the page:
Then Click on "Erase All Content and Settings":
Click on "Continue"
Finally, click on "Finish the download then erase".
If the datas has already been saved/download on iCloud, you can directly press "Erase now".
It's done! Your phone is now being reset and will be totally reset once its restart. It is better to let your phone in charge, so it does not light off during the process.