Tweak the material

Enter the RenderGraph by double clicking on the RenderGraph object visible in the bottom left tab

You can also drag'n drop from the Node List into any window to edit the graph.

In the RenderGraph, select the Surface node.

Using the Property Sheet window (top right), tweak Library/Materials/Surface, like Diffuse > Color for exemple. Re-render to see the result.

The RenderGraph is the place where all edits and tweaks appear. See the User Guide/Render Graph section to know more about the RenderGraph system.