Request for my refund

I ordered an office chair online, as l was informed that it is only available online. It was delivered on Monday the 20th April. I used it the following day and was uncomfortable had had the backrest wasn't stable. I have sent multiple email and made phonecalls that l want a refund. I only received one response that they will replace the backrest, ignoring the fact that l mentioned it is uncomfortable to sit on. I now have to sit on a couch while I work from home and they refuse me my refund. I am willing to take it back to Fourways branch myself as long as l will get MY MONEY. Please sort this out for me. It is unprofessional to be refused my money on an item l do not use.