Where a contract has been reduced to writing, but through a mistake, does not express the agreement of the parties, it may be reformed by a court so as to correspond with the real understanding. This is the more important because of the important principle that a party to a written contract is not permitted in an action founded upon that contract to show by evidence outside the writing itself, that the agreement was not what the written statement shows it to be. He may avoid the contract on one of the grounds stated in the preceding section, or he may show that another later contract, whether oral or written, affects it, but he may not attempt to show by oral testimony that it was something different from the written expression. If the contract was in fact different from what is shown by the writing, and if the other party will not consent to the required change, application should be made to the proper tribunal for reformation.