Go to the building blueprint, like for example the BP_Barracks. Then select the BuildingMesh component in the components panel and in the details panel find the Static Mesh setting and change the mesh there like the image below.

Next in that same blueprint open the Class Defaults and find the BuildingStaticMesh setting and change the mesh there as well, this should be the same mesh as the one you set in the BuildingMesh component like shown in the image below.