Dirac equation is $i \hbar \gamma^\mu \partial_\mu \psi - m c \psi = 0 $
To show its Lorentz invariance, we convert spacetime into $x'$ and $t'$ from $x$ and $t$ and then
$( iU^\dagger \gamma^\mu U\partial_\mu^\prime - m)\psi(x^\prime,t^\prime) = 0$
The question is, how does one show from the above equation the following equation follows?:
$U^\dagger(i\gamma^\mu\partial_\mu^\prime - m)U \psi(x^\prime,t^\prime) = 0$
where $U$ is some unitary matrix for lorentz transformation for $\psi$.