These days it's not as simple as just changing postage. Simple Delivery only approximates the weight, so it's supposed to be OK if the item weighs more than it says on the label. However parcels are still refused so it's usually better to cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
The alternative would be to buy and pay for the correct postage yourself. To mark dispatched and get the buyer's address you still need to generate the original prepaid label (Get prepaid label > Get prepaid label and QR code). It will remain unused. However you cannot add a new tracking number. All you can do is message the buyer with the new tracking details. You would still be able to add the new number to a "Not received" case if the buyer opened one though.
If the buyer paid for the unused label it will be automatically refunded to them in 30 days. If you paid for the original label (ie. have free postage on the listing) you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
