Where a return actually starts
A return is arranged with the retail partner who took the payment, from their order history. This page explains what that involves and how to avoid needing it.
Because the sale happens at the retail partner, the return window, the return label and the refund are all theirs. Open the order in their order history, choose the item and follow the return flow: for the small parcels this generates a label straight away, and for a heavy frame or a complete desk it usually schedules a collection instead, since these are not items to carry to a drop-off point.
Refunds are issued to the original payment method once the item is scanned back in. Nothing on this site touches that process, and no order details are visible from here.
Assembled is generally acceptable, provided it can be taken apart again and the parts are all present. What causes trouble is discarded packaging, drilled tops and cut cables. If there is any chance of a return, keep the cartons flat behind a door until the desk has been lived with for a week.
A damaged item is a different route from a change of mind, and it is worth choosing the right one at the start: damage claims are handled faster and usually do not require the item to travel back.
Most returns in this category are not faults. They are a desk that will not go low enough for the person using it, a top too shallow for a large screen, a monitor arm rated for the diagonal but not the weight, and a clamp that will not fit under a desk with a cross beam. Every one of those is knowable in advance from a published figure.
That is what the tables and the two calculators here are for. Ten minutes with your height, your screen weight and a tape measure against the underside of the desk resolves the great majority of them before an order exists.