Airline changes flights times at short notice

My partner have pre-booked a flight from Cape Town to Jhb departing on 29/11 at 06h00 as we need to be in JHB early for a connecting international flight. We booked and paid online through Mango airlines as they had the best prices as well as an early flight schedule that would leave us enough time to catch out connecting international flights. They just sent an email today notifying us that they have cancelled/changed the 06h00 flight JE 144 to an 08h55 departure - which would only arrive in JHB at 11h00