You need to build the lighting on the production level setting.
Click the down arrow beside ‘build’ at the top area of Unreal Engine.
Hover over the “Lighting Quality” and select “Production”.
Click the down arrow of the ‘build’ menu again and select “Build Lighting Only”.
That will take some time and will remove the “preview” sticker plastered everywhere.
As for reducing the shine from a texture, you need to go into the blueprint for the material by double clicking it. In the Graph editor, add a sigular ‘constant’ node by right clicking and typing “constant” and selecting it from the results.
Attach that node to the roughness of your material by clicking and dragging from one to the other. Give the constant a value of around ‘0.6’.
If needed, experiment with the roughness value.