To revoke a will without making a new one, all you have to do is intentionally tear it up, deface it, burn it or destroy it. If this is done accidentally, then the will is not revoked.
An old will cannot be revived once it has been revoked. If you make a new will (which revokes all prior wills) and then decide you like your old will better, you would need to make a whole new will that replaces the new one and mimics the old one. Otherwise, all old wills are invalid.
