AS, WN, FL and B6 do it for free in the form of a travel credit. You won't get cash back but you can use the credit for another ticket. Most others do it for a fee, generally $150 for a domestic US flight though it can vary.

Once you get outside the USA the answer is actually often just plain "no" on the cheapest fares, even those sold by US-based carriers where comparable fares originating in the USA would have the penalty but be changeable.