Hi. I tried to change my old icloud account on a new one. After reset the settings I need to enter my old Apple ID account and password, that I don't remember.

Hi. I tried to change my old icloud account on a new one. After reset the settings I need to enter my old Apple ID account and password, that I don't remember.

I do not remember it because when I bought the my iPhone 4s in 2011 I didn't understand how it works and asked a representative to set it up for me. The representative helped me and gave a piece of paper with login and password but I lost it a long time ago.

Before resetting the settings I turned off "Find my iPhone" function, but it anyway asks me to enter my old Apple ID and password. I have the original box, documents, receipt. So how can I solve this problem?