MTN IS USELESS

I have had a cellphone contract with MTN for many years. I can't understand how difficult it is to update my address. It is correct on the computer system but every monthly invoice/statement I am sent still has my old address on. I have lodged at least 3 queries that I don't believe go anywhere or to anyone who cares. What does one do? Do I need to cancel my contract? It is up for renewal in about 2 months........