You'll need a laptop / PC for this and use Chrome, that combo seems to work best.
If you have generated an original label with Simple Delivery, then go to View Order details top left of the sold item or More Actions. Go to Postage>>Track Postage drop down arrow and change the tracking.
If you paid for the original postage cancel the label here within 14 days for a refund, it can take 30 days .
Find your label with this link . https://www.ebay.co.uk/ship/lmng/labels-history/seller-hub and to the left of the sales record there is a drop down menu under 'Actions' click on that , then click on 'Void Label'.
@pading_56