Poor quality/ poor service

Please help me I bought a bed at OK Furniture , after few days the mattress started sagging, called them and sent them pictures of the mattress and they said they can see it’s faulty then they did an exchange for the mattress again after few days it’s started sagging and again they asked for pictures and again they said it’s faulty they will change the mattress. The problem is I no longer want that bed I want another but they refusing because they say the base I not faulty, which means they want to change the mattress for the 3rd time.