Failure to change my flight

I’m so frustrated with the service I received I called numerous times to change my flight time for tomorrow to 6 in the evening and I am being sent from pillar to post. Yesterday I was told someone will call to sort it out and it did not happen. Now I’m told to call flysafair to change the time and now I have to pay an additional cost. What’s the point of booking a flexi flight??? I will never use this website again LX7L2P