Refusing to give me another phone or get that checked

Hello everyone, on Friday the 26th of Feb i bought a cellphone( Nokia c1) at Jet store by Lakeside mall. The lady who was a cashier told me that if am experiencing any problem with the phone am allowed to take it back within 7days. Over teh weekend i have been trying to do my school work but the phone is veey slow so today the 1st of March i took it back and they refused to take it back. What to do?