Find the item in your purchases and click on more actions. Then choose 'return this item' from the dropdown menu.
That will start the process.
Returns have to be started within 30 days of receipt.
Private sellers don't have to accept returns.
Knowing what you bought, when you received it and why you are returning it would help.
So, add those details if you have any further questions.
"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)