To tune a parallel resonant circuit to a higher frequency, the capacitance should be decreased. Here's an explanation:

Understanding Resonance in a Parallel Circuit:
A parallel resonant circuit consists of an inductor (L) and a capacitor (C) connected in parallel. At a specific frequency, known as the resonant frequency, the circuit exhibits maximum impedance. This occurs when the reactance of the inductor (XL) cancels out the reactance of the capacitor (XC), resulting in a purely resistive impedance.

Effect of Capacitance on Resonant Frequency:
The resonant frequency of a parallel resonant circuit is determined by the values of the inductance and capacitance. The formula for calculating the resonant frequency (fr) is:

fr = 1 / (2π√(LC))

From this formula, it can be observed that the resonant frequency is inversely proportional to the square root of the product of the inductance and capacitance. Therefore, to increase the resonant frequency, either the inductance or the capacitance must be reduced.

Reason for Decreasing Capacitance:
In the given scenario, we want to tune the circuit to a higher frequency. This means we need to increase the resonant frequency. Since the resonant frequency is inversely proportional to the square root of the product of inductance and capacitance, decreasing the capacitance will result in an increase in the resonant frequency.

If the capacitance is decreased, the denominator in the formula for the resonant frequency will decrease, leading to an increase in the resonant frequency. This adjustment ensures that the reactance of the inductor and capacitor still cancel each other out at the new higher frequency, resulting in maximum impedance.

Conclusion:
To tune a parallel resonant circuit to a higher frequency, the capacitance should be decreased. This adjustment will result in an increase in the resonant frequency, allowing the circuit to exhibit maximum impedance at the desired frequency.