Possessing problem

I managed to possess a vehicle with a firstperson bp, but I want to unpossess it aswell.
how do I make the connections??

You would use Unpossess either from within the vehicle bp or the controller bp. Press a key to exit the vehicle, Unpossess it, spawn a new character, and Possess that one. You probably want to get rid of your existing character when entering the vehicle too, or at least hide it and relocate it correctly when re-possessing.