  We compile an updated list of 38 measurements of the Hubble parameter $H(z)$
between redshifts $0.07 \leq z \leq 2.36$ and use them to place constraints on
model parameters of constant and time-varying dark energy cosmological models,
both spatially flat and curved. We use five models to measure the redshift of
the cosmological deceleration-acceleration transition, $z_{\rm da}$, from these
$H(z)$ data. Within the error bars, the measured $z_{\rm da}$ are insensitive
to the model used, depending only on the value assumed for the Hubble constant
$H_0$. The weighted mean of our measurements is $z_{\rm da} = 0.72 \pm 0.05\
(0.84 \pm 0.03)$ for $H_0 = 68 \pm 2.8\ (73.24 \pm 1.74)$ km s$^{-1}$
Mpc$^{-1}$ and should provide a reasonably model-independent estimate of this
cosmological parameter. The $H(z)$ data are consistent with the standard
spatially-flat $\Lambda$CDM cosmological model but do not rule out non-flat
models or dynamical dark energy models.
