Unfortunately, Non-Flexible, Supersaver or Advanced Purchase bookings simply cannot be amended/cancelled because they are offered at a discounted rate off the standard price. The discounted rate is ideal for customers whose plans are definite with little chance that they will need to make a change.
However, if you feel your plans are likely to change, we highly recommend booking a fully flexible option at a slightly higher rate, it is often only a few pounds more expensive and could save you money in the long run.