Why do you want to return the item?
If it is a case of the item being faulty or not as described, the seller will have to arrange for the return.
If you have changed your mind about a purchase, you may need to research carriers who will take the item you need to send.
Either way, you start by openeing a return case, as you would with any item.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)