How to click on a 3d object and put an image (texture) in this and other objects?

I'm developing an application for hololens but I have almost zero experience in Unity and C # oriented games.
So I need the maximum help, putting some doubts here in the forum and studying.
How can I click on a cube and change the png image in it and other cubes?
Where do you advise me to study to help me in building the application and how?
Thank you for your attention