Poor service delivery

I bought a Nokia c20 from Pep cell at Mabopane Central city on the 18th of July, the phone is not working properly, some of it's applications are not functioning properly, they are very slow. Today on the 1st of August I returned it and the cashiers decided to tell me where to get off. They said I know that the phone is slow and there's nothing they can do, and the phone is still under warranty. I really need to return the phone and get my money back.