You can't change a Simple Delivery label.   You can cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist with different postage for the buyer to buy and pay again.   

You can opt out of a courier on here:
https://www.ebay.co.uk/ship/prf

When you relist you may be able to select "Oversize" on the package size pop-up.   Then opt out of SD and buy your own postage.

Or list with free postage and mark dispatched by opening the SD label (Get prepaid label > Get prepaid label).  To get the buyer's address you have to print label.  It will remain unused and you can claim a refund for an unused label here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller

If you can't be bothered with all that you can just buy and pay for your own postage.   You cannot change the tracking and payment may be held for 14 days.