You'll need a laptop / PC and try Chrome that seems to work well, not the app or mobile browser, then you can add your new tracking details manually.
Go to "Orders Paid & Dispatched"
Click on "View order details"
Within the "Postage" section there's a small dropdown arrow next to "Track Package" - click on that.
Click on the "Add tracking" option
A box will come up with the current tracking number . Delete that and replace it with your tracking.
Also,
If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days
If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling.
