Can anyone tell me how to edit just a single wall within an imported floor plan?
You’d get more responses if you post a screen shot and or describe a bit more what you want to do or what the problem is.
As @eric-s says, no one can really help you without either more info or, best of all, seeing the file. Much will depend on things like whether the plan is a 2D object or part of a 3D one, and how things have been grouped, if at all.
Your question is a bit like asking how to fix an engine that won’t go. And asking people who aren’t there and can’t see it!