The buyer can do nothing their end, you can manually change the tracking on the order.
Use a laptop / PC and try Chrome that seems to work best, not the app or mobile browser, then change the tracking this way:
- 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.
@drumcounty