They will usually send you the plastic bag an labels to send the phone back after you have received your new phone, they then give you 14 days to actually transfer everything over to your new phone and then package the old phone up and return it to them.
You can check the return of the phone status online and if there is any problems they will email to let you know (as last time there was a glitch that said they hadn't received the phone when they had, but a quick email to Samsung sorted the situation out)
"
NEXT STEPS...
You should have received a Samsung Switching Programme return pack soon after you got your new Samsung device. Please use this to send us your old Galaxy S8 Plus - G955F. If you haven't received the return pack please use the link below and another will be sent out you to.
RETURN PACK...
Your return pack will include freepost packaging for your old device and instructions on how to post it back to us.
Follow the simple postage instructions and send us your device within 14 days.
Upon receipt our experts will check your device and an email will be sent to you confirming that your switch is complete."
This is part of the email they send you and like I said you can track everything online as they give you a tracking number.
