Very unsatisfied consumer ?

I have been with vodacom more than 3 years now...Since last year November I have problems with them, I bought a new laptop from them end of October 2017 but it was faulty I couldn't use it so I battled for over a month for them to exchange it as I am not paying for a new faulty device. So the took it in and I waited almost 3 months for a replacement but every month they took my installment. I demanded to be credited as it wasn't my fault. The manager Warren told me they have approved the credit but it's now October again and still I am waiting. I have changed banks so I went in to change my banking details for my debit order and now after the 3rd time I went to see them it still hasn't been changed. And also every month my subscription amount change!!! I'm fed up now and I need answers ASAP... PETER PLEASE HELP...