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.

https://www.airliners.net/forum/viewtopic.php?t=748133

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.