  The ringdown phase of a binary black hole merger is modelled by the
quasi-normal modes of a perturbed Kerr black hole. According to the black hole
no-hair theorem, the emitted ringdown spectra are constrained by the mass and
spin of the remnant black hole and thus offer an excellent test of the
Kerr-nature of black holes. As a parameterization of beyond-Kerr effects, we
employ the Johannsen-Psaltis metric ansatz. We analyze the ringdown of two
binary black hole merger events- GW150914 and GW190521 to constrain the
deviation from Kerr. We find that both events are consistent with the Kerr
metric and due to the larger signal-to-noise ratio and the presence of the
additional subdominant mode in the ringdown phase, we find a factor of ~2
improvement in constraints on the deviation parameter in the case of GW190521
as compared to GW150914. Moreover, we find that the deviation parameter is
anti-correlated to the spin of the remnant black hole. We also explore the
effect of priors for other physical parameters on the Kerr deviation.
