Usually a piezoelectric crystal subject to a time varying signal will resonate at the drive electrical frequency with the highest amplitude taking place at one of the two possible natural resonant frequencies.(series and parallel RLC Values) In short, the higher the drive amplitude, the greater the crystal amplitude change. However, other than permananently changing the crystal dimensions,  to vary the principle resonant frequency is not so easy  -  changes can be accomplished by changing the temeperature at which the crystal operates and is most effective with polycrystalline (e.g PZT) or intrinsically temperature resistant crystals such as Quartz. SiO2. Natural piezoelectrics such as Rochelle salt (sodium potassium tartrate) are not ideal for this. See my book:- BC Blake-Coleman  'Approaches To Physical Measurements in Biotechnology' pp 96-104.  .   

This should not be a problem to change frequency and amplitude of PZT actuator. Yes, you can simply change them by a change in the power amplifier supplied to the PZT. You may get help from one of my published articles herewith. https://www.researchgate.net/publication/261720936_Robust_H-infinity_Control_of_Hysteresis_in_a_Piezoelectric_Stack_Actuator

Hope this helps - Ning

Article Robust H-infinity Control of Hysteresis in a Piezoelectric S...

If you are talking about the resonance frequency of the piezo actuator, that can be changed by adding mass/stiffness to the actuator. 

If you are not talking about resonance, but the actuation frequencies, then you can use any function generator with an amplifier to modulate the frequency and amplitude below the first resonance of the piezo. I suggest using a dSpace or National Instruments input/output board interfaced with your system so that you can generate signals in simulink/matlab and directly feed it to your system. 

Sure! I agree with Sanidhya's statement about the difference between resonance and the actuation frequencies, as I have had another paper in terms of the PZT's resonance, see the RG link herewith; https://www.researchgate.net/publication/260586854_Robust_H_control_in_nano-positioning?ev=prf_pub

Good luck - Ning

Article Robust H∞ control in nano-positioning

Yes, but mass loading a piezoelectric crystal changes the damping factor which not only alters the resonant frequency but also requires more drive to establish any required actuation frequency.  Operating temperature defines the effective 'stiffness'  and intrinsic polar separation,  thereby changing resonance without the need to ,mass load the crystal.  

Hi guy, may I know any method to vary the frequency of the piezoelectric generator as shown in the attachment?

Thank you and appreciate your help.

If you are mass loading the crystal the resonant frequency will drop. The physical properties of the crystal will determine its base resonance so the more mass applied the lower the frequency and amplitude of any of the resonant conditions (see above). In short, the crystal has fixed properties and there is little can be done to make it operate as a universal resonator.