Once you have more than one pilot, in Pilot Dossiers, just scroll through pilots using Previous Pilot / Next Pilot arrows, and "Select Pilot" to make that the currently viewed pilot the "active pilot". Be careful in QC as you fly with your current pilot. If he dies that's it. Or create a special QC pilot (one for each nation if you want to fly all types of aircraft in AC) and select him each time - or in QC you can tick the box "Pilot Never Dies" to cheat death (or even set it for all pilots in all modes, in workshops, more settings, under Outcomes.