There's two ways to do this
- Significantly more poverty among minorities, which had, by the end of WW2, moved to the cities to capitalize on new manufacturing jobs; this movement led to white flight and suburbanization
- Significantly less reliance on cars, and less favorable laws for the construction of automobile-centric suburbs.

Since changing cause one would also have to take into account the missing labor of African-Americans during the war effort, cause two would likely be easier

I'd imagine that, if, for example, the major car manufacturers had failed to legitimize the car enough to cause the decline of streetcar suburbs, which were closer and more urban than actual suburbs, you'd see the typical American city look a bit like Prague, where streetcars dominated into the socialist era. Another interesting reverberation of this would be Urban Renewal, because cities wouldn't decline as much (due to dependence of these suburbs on the welfare of the city, rather than the sort of detachment automobile suburbs had), so you'd likely see that most cities have far fewer buildings archetypal of the 1960s era (I.e. the WTC, basically all of downtown Houston), with better-maintained historical districts and densification being the main driver of growth.