Thanks for your post and questions surrounds our engineer slots.
Some kind of appointments will not be able to be changed due to the type of appointment they are for. These are mainly if we need to complete work or tasks before your appointment.
If it's just a usual fault engineer then you should be able to change this online as long as it's not after 4pm the day before the appointment.
The slots are updated as often as they can be but earlier appointments can go very quickly as our agents in the faults team may be booking appointments at the same time.
Taking a look at things, there are a few earlier appointments available so I've sent you a private message to check if these are any good for you. Just click on the little plum envelope at the top right-hand side of the page to access your inbox.