You can mark dispatched by opening the original Simple Delivery label (Get prepaid label > Get prepaid label and QR code). It will remain unused and the cost will be automatically refunded to the buyer in 30 days (assuming they paid for it).
Or if you paid for it (ie. you had free postage on your listing) you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
The option to change tracking number has been removed. You can message your buyer with the tracking details. If necessary you would be able to add the new number to an "Item not received" case if one was opened. Payment will be held for 14 days.