Hi.
I’d just like to start by saying I’m a noob at Unity.
I have placed multiple objects of the same type (the same wall) in my scene, but I have since updated the model that looks considerably better.
My issue is, how do I now swap out the old wall for the new one without going through each wall, deleting it and placing the new one down each time?
Thanks 
You want to read up on prefabs.
Does it have to be a prefab before (so do I in this instance I still have to change all the walls?)
This helps out alot for the future, so thanks!
What I would do is change one of them, make it a prefab and then instance that prefab as many times as you have walls in your scene, then one by one copy the transform component’s values of the old walls and paste those values on the new prefab instances, then delete the old walls.