This is a very "Don't do this" kind of situation but you can force the computer to switch to a different law by pulling circuit breakers for certain instruments. Typically though it will happen automatically when the computer detects sensor failures.
Changing the law of the aircraft is something you REALLY do not want to do. It's a "The Airplane broke real bad, do that pilot thing!" situation. Especially on a fighter jet with relaxed stability.
Some fighters have an "EFCS" switch which will switch laws. This can be used during "I'm going to fly into the ground" or "I'm losing this dogfight" situations. Typically this means you are going to be scrapping the airframe soon one way or another.
Yeah, in the Su-27 Flanker that is exactly what it is. You change the flight controls to direct control so you can do things that will put the airplane in an unsafe condition. I think it's a switch in the Flanker as well.
In jets like the F-22 Raptor it will just "cobra" with stick input. They will dump the wing controls automatically to unload the wings, and use thrust vectoring to reach absurd angles of attack. The F-18 Hornet uses a paddle switch which will increase the G-limiter and allow spin recovery.
As I understand it, there are no switches because pilots aren't supposed to switch modes, but if necessary, pulling certain circuit breakers will disable subsystems whose failure triggers alternate law. And AFAIU it is documented which breakers are "least unsafe" to use that way.
