It depends on your last date of attendance. As a Home/EU undergrad or Postgrad taught student, you will get a full refund if you withdraw before the end of the first week of teaching. If you withdraw later than this you will be liable for at least 25% of the total fees.
The rules differ somewhat for international and postgrad research students. Please consult this page for more guidance http://www.swansea.ac.uk/undergraduate/fees-and-funding/tuition-fees/refunds-policy/.
For questions, help and advice with the financial implications of leaving the University please contact MASO: http://www.swansea.ac.uk/money-advice/