  By inverting the observational data of several neutron star observables in
the three dimensional parameter space of the constant speed of sound (CSS)
model while fixing all hadronic Equation of State parameters at their currently
known most probable values, we constrain the three parameters of the CSS model
and their correlations. Using two lower radius limits of $R_{2.01}=11.41$ km
and $R_{2.01}=12.2$ km for PSR J0740+6620 obtained from two independent
analyses using different approaches by the Neutron Star Interior Composition
Explorer (NICER) Collaboration, the speed of sound squared $c_{\rm QM}^2$ in
quark matter is found to have a lower limit of $0.35$ and $0.43$ in unit of
$c^2$, respectively, above its conformal limit of $c_{\rm QM}^2<1/3$. An
approximately linear correlation between the first-order hadron-quark
transition density $\rho_t$ and its strength $\Delta\varepsilon$ is found.
Moreover, the presence of twin star is deemed improbable by the present work.
