  Radio relics are elongated sources related to shocks driven by galaxy cluster
merger events. Although these objects are highly polarized at GHz frequencies
($\gtrsim 20\%$), high-resolution studies of their polarization properties are
still lacking. We present the first high-resolution and high-sensitivity
polarimetry study of the merging galaxy cluster CIZA J2242.8+5301 in the 1-4
GHz frequency band. We use the $QU$-fitting approach to model the Stokes $I$,
$Q$ and $U$ emission, obtaining best-fit intrinsic polarization fraction
($p_0$), intrinsic polarization angle ($\chi_0$), Rotation Measure (RM) and
wavelength-dependent depolarization ($\sigma_{\rm RM}$) maps of the cluster.
Our analysis focuses on the northern relic (RN). For the first time in a radio
relic, we observe a decreasing polarization fraction in the downstream region.
Our findings are possibly explained by geometrical projections and/or by
decreasing of the magnetic field anisotropy towards the cluster center. From
the amount of depolarization of the only detected background radio galaxy, we
estimate a turbulent magnetic field strength of $B_{\rm turb}\sim5.6~\mu$Gauss
in the relic. Finally, we observe Rotation Measure fluctuations of about 30 rad
m$^{-2}$ around at the median value of 140.8 rad m$^{-2}$ at the relic
position.
