Both is true. Future is both fixed and is also changing.
It’s counterintuitive but it becomes apparent when we see that our destiny is fixed to… our karmas or actions, isn’t it!
Thus destiny is fixed and destiny also changes as our karmas change.
For example, a person has been a heavy smoker and his lungs are in bad shape. What is his future right now?
That he will die of cancer. But if right then he starts living a healthy life, he has set another chain of actions or karmas that will have its destiny. Now this new karma will negate some of the bad karma and the person might live a little longer.
Hence the consequence of karma is fixed but another karma can effect the outcome of the previous karma.
Thus comes the idea that we should do good acts to rid us of the sins of our past acts.