Parent exercising access rights visad out of UK help to complete

We have followed all instruction and made priority payment for parent of a child under the age of 18 in the uk, British citizen. They have sent us to TLS contact center. I wrongly choosed Premium Location but not confirmed date. I want to cancel Premium location in Barcelona and book appointment for Madrid for normal date. Anyone could help us with it? Could you explain how to do it? Is there any form to cancel and book in Madrid with the same application or do I have to prepare a new application? Could I complete a new one and give reference of the payment? I do not want to loose time of asking refund of the payment to be able to change location from Barcelona to Madrid. Any help? Please.