Activation Lock

Please I need your help solving my problem, A long time ago my brother bought a used iphone 6 plus from one of the stores and it was working fine until a few days ago when I wanted to download an application for the school, I could not download it because the phone had to be updated to verson 13, After that I connected the phone and I used the itunes program to update it, But after updating it the phone started asking for an Email for activation but it's not the same Email that I used in the apple store to download applications, So it became a problem for me and I went back to the store from which we bought the phone to ask him about the Email but he said that he did not know the Email because the phone was used when we bought it, So please guide me how we solve this problem.

Thanks and Regards,

Rita