During the exit interview, don't take the charges against you too personally. Find out whether there is anything at all that is actually your fault and resolve to do better at it next time. Prove to HR that they were wrong because you came across as a most reasonable person. Obviously, tell them you are shocked and dismayed and you disagreed with their overall assessment. But you see the futility of trying to convince them they may have a bigger problem with your boss, and you wish them good luck and the very best.