I was due to fly out on the 19th June as well, I did mine via manage my booking. If you click on your booking you should see all of your itinerary scroll right to the end and it says cancel my booking and change my booking in blue - click change your holiday then you will need to search for a holiday and when you have the holiday you want click to confirm. Make sure you do it in the website and not the app. They did have a few website issues though so maybe try tonight and again in the morning