  We study, analytically and numerically, the structure and evolution of
relativistic jetted blast waves that propagate in uniform media, such as those
that generate afterglows of gamma-ray bursts. Similar to previous studies, we
find that the evolution can be divided into two parts: (i) a pre-spreading
phase, in which the jet core angle is roughly constant, $\theta_{c,0}$, and the
shock Lorentz factor along the axis, $\Gamma_a$, evolves as a part of the
Blandford-Mckee solution, and (ii) a spreading phase, in which $\Gamma_a$ drops
exponentially with the radius and the core angle, $\theta_c$, grows rapidly.
Nevertheless, the jet remains collimated during the relativistic phase, where
$\theta_c(\Gamma_a\beta_a=1)\simeq 0.4\theta_{c,0}^{1/3}$. The transition
between the phases takes place when $\Gamma_a\simeq 0.2\theta_{c,0}^{-1}$. We
find that the "wings" of jets with initial "narrow" structure ($\frac{d
\log\,E_{iso}}{d\log\,\theta}<-3$ outside of the core, where $E_{iso}$ is
isotropic equivalent energy), start evolving during the pre-spreading phase. By
the spreading phase these jets evolve to a self-similar profile, which is
independent of the initial structure, where in the wings
$\Gamma(\theta)\propto\theta^{-1.5}$ and $E_{iso}(\theta)\propto
\theta^{-2.6}$. Jets with initial "wide" structure roughly keep their initial
profile during their entire evolution. We provide analytic description of the
jet lateral profile evolution for a range of initial structures, as well as the
evolution of $\Gamma_a$ and $\theta_c$. For off-axis GRBs, we present a
relation between the initial jet structure and the light curve rising phase.
Applying our model to GW170817, we find that initially the jet had
$\theta_{c,0}=0.4-4.5~\deg$ and wings which are consistent with $E_{iso}
\propto \theta^{-3}-\theta^{-4}$.
