This probably needs a telephone call unless you kept a record of the change conditions at the time of booking. These are sometimes still visible in Manage My Booking, but the full version comes up during the booking process and it's important to keep a record of this. In the absence of that you need to call the contact centres. They often are busy, but not all the time. If you are calling the UK number then I suspect it will be getting quieter at lunch time and then again around 4pm. It's not always jammed, as we can see from other posts, but there can be overloaded lines at some parts of the day.

The voucher process is now over, so that's unlikely to be offered to you, and if you go for a refund, well again it depends on the fare rules. But if you are trying to make an inflexible booking a flexible one then that can be expensive. So rebooking to new dates is better, usually, but the booking will continue with the same change conditions after that, so any further changes will come at a similar additional charge.

Depending on where you live, some Points of Sale can be more flexible.