Very angry customer

My son bought a bicaycle in dec 2020 not even two weeks it broke we went there to tell them they told us that it can only be fetched in january 2021..they came to fetch it and went and fix it at mikka hardware not even a week thereafter it broke again..we went there to tell them they must cancel the deal cause that bicycle will give problems forever and a day and we do not want to pay for a new broken bicycle..the managaer told us she will phone headoffice and they must tell her what to do..i told her that your headoffice cannot tell me to keep a broken bicylce and pay for it..up to today they still did not cancel the deal and we keep on have to go in and out the store to try to resolve this problem..i do not have the time and energy to go there everytime explaining myself over and over again..please help in this matter