@LAPP best best is to try and contact customer service first, as you need to find out which office your application is with. They often internally transfer applications to different offices without letting you know, so I would advise checking before sending anything. Depending on what stage your application is at, they may also advise you to contact your local office as opposed to the one dealing with your application.
You need to be travelling within 2 weeks time or they wont look at your request. Then it’s a case of emailing your full name, DOB, address, application reference number, contact details (email and phone number), date of travel and evidence/confirmation of flight details to [relevant office]UpgradeTeam@hmpo.gov.uk.