Mango is useless

Booked a flight for the 23rd of April at 17:55, Received a sms on the 19th stating that the flight has been postponed to 20:10. Today the 20th of April I received a call saying that the flight is fully booked and they only have availability for a much earlier flight or the next day. I specifically chose that flight because we are working on the 23rd and would fly after work and have an appointment the next day. They are refusing a refund and want to issue me a voucher.