Where a return starts
On the order page of the partner store you bought from. That store took the payment, holds the record and sets the return window, so it is the only place a return can be opened. There is no form here and no address to send a figure back to — we never had the parcel.
Open the order, choose the figure, pick a reason, and follow what the store offers. Most will either send a label or arrange a collection; which one depends on the seller and on why the figure is going back.
What usually qualifies
Arrived broken, arrived with a part missing, arrived as the wrong figure, arrived in a box that was clearly crushed in transit. These are straightforward and resolve quickly, especially with photographs taken before anything was unpacked further.
Changed your mind is a different category and depends entirely on the store's own policy, which the listing states before you pay. Read it there rather than assuming — it varies by seller even for the same figure.
Warranty on a figure
A static display figure has no mechanism, so warranty here means manufacturing defect: a mis-moulded part, paint that was wrong in the box, a base that will not hold the sculpt it was made for. Raise those exactly as you would a damaged delivery, through the order page of the partner store you bought from, with photographs.
Sun fading, a lean that developed on a warm shelf and dust that was cleaned off with a household spray are not defects, and no seller will treat them as such. The delivery page explains how to avoid all three.