You can't change it unless you buy and pay for the postage yourself and then have to change the tracking number on ebay.
The best thing is to cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list). Opt out of using Evri here:
https://www.ebay.co.uk/ship/prf
Then relist with Royal Mail postage for the buyer to buy and pay again.