CUSTOMER may early terminate this Agreement with respect to a Supported Software product as follows: (i) giving a thirty (30) days prior written notice to NCR and certify that CUSTOMER has stopped using the copies of such Supported Software product; or (ii) if NCR has changed the Software Services for the Supported Software product, by written notice to CUSTOMER prior to the effective date of the change. In this regard, NCR shall return the unused portion of fees and payments to CUSTOMER on pro rata basis within fifteen (15) days from the effective date of the early termination otherwise, NCR shall pay interest on the unused portion which must be returned at the rate of 15% per annum to CUSTOMER until the amount of unused portion is fully refunded to CUSTOMER.