Assuming you used ebay's Simple Delivery (which you will have done unless you actively opted out) you cannot cancel the label. You'd have to cancel the transaction with buyer's agreement (via the dropdown menu arrow on right of item on your orders page) and relist with the other postage. If the buyer paid a refund would be issued in the process, and then they have to buy and pay again.
If you want to opt out of SD you do that during the listing process. If the Custom Postage option is there select that. If not click on the postage options icon on the right of the postage section and remove details of parcel weight and dimensions. Then you should be able to add what postage you want.
If you don't want to cancel you'd have to buy the second postage yourself and contact ebay to get the tracking number changed.
