I did this yesterday. It’s not possible to change online but the telephone agent was pretty helpful. He put me on hold to work out whether it would be cheaper for me to cancel and rebook or amend the booking.

To amend the booking there was a £170 change free plus the difference in cost of the holiday itself. (£170 was the amount of the deposit I had already paid.)

In my case there was only a £20 difference between the two options (it was cheaper to cancel and rebook). The chap mentioned that sometimes there can be a significant difference between the two. My trip became much more expensive, which I was expecting, as I was switching from the third week in January to a May bank holiday weekend.