There is no button or drill that the crew perform to "select" a different Law specifically... the aircraft systems determine which Law you are in.
There are drills / buttons / levers that have the effect of changing the Law e.g. lowering the Gear in Altn law will put you in Direct law... but you lower the gear so you can land, not to effect the change
There are even some drills where you deactivate systems etc. for the purpose of a law change.
In general, the law changes are transparent to the pilots in flying terms, and I can see no great need to wish to fly in 1 Law or another...
NoD