How is an airplane able to fly upside down?

The lift force provided by the wings of an airplane can be adjusted by changing its angle of attack. It is the angle of the wing of the airplane with respect to the horizontal at which it approaches the onrushing air layers. When the angle of attack is adjusted large, the speed of the air above the wing is larger than below it. According to Bernoulli’s equation, the pressure above the wing is lesser than below it. So, a larger lift force acts on the wings.
Thus, airplanes can be flown upside down by properly adjusting the angle of attack of its inverted wings. In doing so, the inverted wings of the airplane provided the required lift force to support the airplane in flying upside down.