Since you haven’t explained I’m gonna assume that you’re talking about the player’s pawn, and not AI’s.
In essence, after the event for the button was fired, first unpossess the current pawn, then destroy it, and then spawn a new pawn from the class/blueprint that you want, and then move it to the same place and possess it. Almost all the nodes has names exactly the same as what I said you should do, just make sure you’re doing all this in your player controller BP and not another class. (Level Blueprint could also work but is not as straight forward I suppose)