Trying to swop damaged goods and told no

I bought the versed motion tablet on a saturday 2 weeks ago from Jet in Midrand. That Thursday the tablet screen wasn't working - was frozen and I couldn't do anything to it. I went in and swopped it for another one. This Tuesday the screen again was frozen and wouldn't switch on so I took it back and they said they will swop it for another one so I declined as its clearly a fault with the tablet. they did not want to give my money back and I then asked if I could instead put in extra money to get another brand which was agreed too. I did not have cash with me and I asked if my husband could come after work to give the extra money and it was agreed this was fine. My husband then travelled all the way to Midrand from Primrose to get a new tablet and when he got there they refused the transaction saying the screen is working again and the warranty period is up (surely the new warranty should start when I got the new tablet)now the screen is acting up again.I cannot travel each week as I am a domestic worker and I do not think it is fair as the tablet isn't working properly and Ingrid agreed to it and then declinded.what do I do? I paid for a tablet and it should work