The prepaid return labels are located directly beneath the original labels and in the same label pouch.
You open the purple flap, pull out the top label and leave the bottom label (which is the prepaid return label) in the pouch.
Make sure the NEW barcode is showing clearly. If any stickers are covering it (especially stickers that have barcodes on them) they MUST be peeled off.
Otherwise FedEx will simply deliver the box right back to you. (If their equipment can’t ‘see’ the new prepaid return barcode, They will return it to you.)