You can't change it.
Once you have marked the item as 'paid' you can't mark it as unpaid.
You may have to contact Ebay to ask them to change it so your buyer can pay you.
Really, as all payments have to be made through Ebay, you never need to mark items as paid.
"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)
