Film bulk acoustic resonators (FBARs) based on aluminum nitride (AlN) and scandium-doped aluminum nitride (AlScN) exhibit tremendous application aspects in the radio frequency front-end due to achievable high-frequency characteristics, superior thermal performances and compatibility with harsh environments. Delicately controlling the resonant frequency (f_s) of FBAR is essential for integrating filters or modules. In this work, we provide a practical feasibility in adjusting f_s of AlN and AlScN FBAR using external direct current electric bias (E_DC). When applying a negative E_DC (the direction along the reversed c-axis), f_s shifts to a lower frequency, whereas a positive E_DC brings a higher f_s. In order to extract the equivalent values of the stiffness coefficient (c₃₃), piezoelectric coefficient (e₃₃) and dielectric constant (ε_zz) of AlN and AlScN piezoelectric materials, we adopted the electromechanical equivalent Mason model. The results show that the equivalent values of c₃₃ increase with the change of E_DC from negative to positive, and, on the other hand, those of e₃₃ and ε_zz decrease. Our work provides a systematic investigation on the electric field-influenced stiffening effect of AlN and AlScN piezoelectric films and opens a feasibility for frequency-tunable resonators.