It’s currently not possible to change a goal date after it has already been set.  However, deleting and re-creating the goal for the new date will work.  There will likely be some changes to your training in the short term, but as long as the goal is still some weeks away, the changes should not be major.

One thing that could happen is that your periodization schedule will move a bit.  The best thing to do is simply follow the new schedule - it’s still optimized to get you to your goal in top shape.