ah nvm i found it. I'll copy and paste the instructions for those who couldnt find it either:
Extract the archive then copy and paste its content into {GameInstallationFolder}\LifePlay\Content\Heavy\StreetMap
So that {CityName}.uasset and {CityName}.uexp go inside StreetMap folder, and another (much heavier) pair of {CityName}.uasset and {CityName}.uexp go inside StreetMap\Static subfolder.
(For those curious: the first pair holds the map data, while the second pair holds the 3d shapes of the map itself)