Thanks for the help.
It seems that a few of the tips shown in the videos don't actually match with reality. The contextual menus (on right click) do not show what the video suggests they should show. Some are not displayed, and some are greyed.
In the end I saw one video suggests that I should make the edit in the camera view: Select the object that has the desired material, and then the required options appear. It worked.
Very difficult to find.