You are going to hate this, @Rebecca-and-Sean0, but you cannot remove the penalty and the only way to avoid it in future is not to cancel on people.
Was it an extenuating circumstance? An "urgent trip" would not be one, and you do not have to say here why it was urgent, but if it qualifies (see here) you could call Airbnb and plead your case. If an extenuating circumstance arises in future, get Airbnb to do the cancelling so you are not hit with a penalty. Hope that helps a little.