It will switch naturally during the day several times, as long as you get enough rest, excercise, don't smoke, etc.

It's actually very easy to switch it. You can lean to towards the side (think yoga triangle pose) that has the open nostril.

You can place pressure in the corresponding armpit for the same effect.

Medidation, concentration, certain ways of breathing will all also affect the flow.

I'm sure you can determnine the various other physical ways, based on these, to swap nostrils.