A modification comes with a cost.
You can modify your date and/or your destination up to 4 days before departure. For short and medium-haul flights this comes with a cost of € 50.00 per person and per leg. For long-distance flights (Mexico, Dominicaanse Republiek, Jamaica, Zanzibar, Cuba, Mombasa and USA) it comes with a cost of € 120.00 per person and per leg.
If the price of your new flight is higher than the one of your original flight, you will have to pay the price difference in addition to the modifcation fee. If the price of your new flight is lower than the one of your original flight, there will be no refund. However, the modification fee remains unaltered.
A name modification also comes with a cost, see Can I modify a name in my booking?