Unfair treatment a consumer

I bought a tablet at hi fi corporation at waterfall mall in Rustenburg. The tablet lasted for 3 months then started malfunction. I Itook it back took and exchanged it for onother one, again this new one lasted for about three months and stopped working. I took it back only to find that they did not have stock of the same product. So I was forced to take Connex tablet, I had to add some funds as it was more expensive than the previous one. Again it functioned for 2 months. When I took it back the manager told me that must take it back to supplier. I told him that i dont need their tablets as it had shown that that they not good. He told me that sending to supplier is a procedure when it comes back he will refund me. After 2 weeks they called me to come and collect my tablet. When i told him that the agreement was a refund. The refused saying that he never promised that. I have not collected the as I know its gonna give some problems. Please help me what must I do.