You can only change modules in the first three weeks of a semester and you must obtain approval from the School(s) teaching the module(s) that you wish to change and you wish to add.  Your Personal Tutor or another staff member in your School will be able to advise you.  You can only change modules in accordance with specific regulations that relate to your programme and year.

https://intranet.cardiff.ac.uk/students/your-study/changes-to-your-studies/changing-modules