Try expanding these dropdowns under Details:
Transform, Static Mesh, Materials, Lighting, Rendering, Actor.
Double-clicking the object you placed opens it in the mesh editor, and right-clicking it in the viewport (main editor) opens a list of options to do different things. To modify the material that’s currently on it, select the object > expand the Materials category under Details > left-click the spyglass icon to find it in the Content Browser (list of files at lower part of screen) > double-click the highlighted material (or right-click and select Edit…).