Has Allah created human beings along with their destinies. Can duas change someone’s destiny? if so how can this change arise when someone’s destiny has already been written by Allah who no doubt knows everything?

The destiny is knowledge of Allah Ta`ala. Whatever will come, has come is in the knowledge of Allah. We are prescribed to ask dua and ask for betterment in this life and in the hereafter. As such the result of dua is definitive. There is a concept of Taqdeer Mu`allaq that define destiny as flexible to certain extent and changeable. Hazrat Ali (r.a.) described destiny as one person being able to lift one feet up and the other feet need to support him on earth. That is part of it fixed and part of it in our hand. But in fact the choice to be taken of what is in our hand also is known in the knowledge of Allah. As such all destiny are fixed by Allah. But the holy prophet requested that we make effort and make dua and hope good from Allah Ta`ala. Destiny is used for the sad past issues so that we should not regret on them.