Since the energy spreads out in a spherical shell, the energy density decreases as the square of the distance from the source. That's your amplitude decrease.

How does increasing frequency affect amplitude?

When the frequency of the wave is increased, the amplitude decreases.

When amplitude decreases what happens to frequency?

The amplitude and the frequency of a wave are independent of each other. The amplitude and the frequency of a wave are equal. The amplitude decreases with an increase in the frequency of a wave.