Is it possible to modify the data once it is written in a block? Why?

Hi Mauro ,
Great question my friend. However the answer is “it’s not a yes or no issue”: short answer, yes but it’s not something good to do.
Long answer, Blockchain is a democratic network based on resources meaning the if your computational power (CPU or GPU) is greater than the rest of the network combined ,then the Blockchain considers you as the majority (check : 51% attack in Blockchain ) at that moment you can change what ever block you wish to change then re-mine your way back the correct block and timestamp and that’s what we call the Double spend attack