Apple id

Hello everybody!

I would like to know what to do if the apple id is forgotten?

I will explain in more detail

A few years ago, my Uncle gave me an iphone, after which he left for Germany, and I myself from Russia, and a few years later I wanted to give this iphone to a friend.I reset the settings and untied the apple id, after which I gave it to a friend,he started setting it up with me.But there is such a problem that he writes that this iphone is tied to an apple id,and this apple id is my uncle.I don't have any contact with my uncle right now, and I don't know where he is.A friend was upset about the situation.

Please help me)