After you have submitted the project agreement to the Board of Studies you can only change the project title. However, remember that you are not assessed based on the problem statement as it appears in the project agreement, so it is fine to put a revised problem statement in the finished project, if you can agree on it with your supervisor.
If you have made substantial changes to the problem formulation, method or the like, you may want to describe them in an appendix or a summary about the project process in the finished project.