How should I behave?

I was accused for the 3rd time of using an engine, which is absolutely not the truth.
They also wanted my real name, DWZ / ELO or a chess club as proof.
You can read about my chess career there lichess.org/forum/general-chess-discussion/my-plan-to-become-a-gm

For the last 6 of today's online chess games, my chess coach sat next to me and please believe me,
he didn't give me any help during the game.
He is an old and very honest "GM" a bit like my grandpa.

How do you react best when you are accused of being a cheater?

If someone is wrongly accusing you just ignore him, if you weren't cheating you won't get banned

Go up and ask them WHY you think they are cheating.
Then tell them they are wrong if their reasons are invalid.
Make sure to do all of this in a kind tone.

Hello,

If the accusations happened on Lichess, either in public chats or private messages, you may use lichess.org/report.