How do I air out feet on an airplane?

Shoes off. Make sure your feet are clean when boarding, then when seated, take your shoes off for the flight.
Sweaty Feet. If you are someone that experiences increased perspiration in shoes and feel you need to allow them to air periodically, i recommend that you wear shoes that accommodate a natural fiber sock (such as cotton) and are easy to take off and put back on. I find that clog style shoes (such as dansko or sanita) are a breeze in the airport and on airplanes for my patients.