There is no cancel button on your order page. Cancelling is done by the seller or by support, so you need to ask one of them.
Before the seller posts it
This is the simple case. The seller cancels, the item goes back on sale automatically, and your refund follows the usual route.
After the seller posts it
Once the parcel is with the courier it cannot simply be called back. This becomes a return or refund case instead. Read the refund and return article, which covers what to do.
Ask quickly
Sellers are expected to post within two working days. The window where cancelling is easy is short, so message the seller as soon as you decide.
Whoever cancels has to give a reason, and it is recorded on the order. Say clearly why you are asking, because that reason is what support reads later if there is any argument about it.