Go to Selling/Orders, select Postage labels on lefthand side, then click on Actions dropdown menu for the label and select cancel or void Label.
If the other tracking number is still there see if you can use the Edit Tracking link next to the item. If not contact ebay and they will change it for you. You can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
If the other tracking number is gone after cancelling the label add the new one and mark dispatched via the dropdown actions arrow for the item on your orders page.
