It's simple to switch between modes, just in ASA global configuration mode enter fxos mode appliance or platform.Please be aware that when you are switching between modes you will lose running configuration and device will be in default configuration...
If you are in appliance mode, FXOS is used only for advanced troubleshooting. To get into FXOS from ASA you need to enter "connect fxos".From my point of view, platform mode is pretty annoying since you need to configure device on two different place...