Tiling/Offsetiting Texture

I have a very thin cube (480,340,1) which i want to texture so that its front face is covered with a different part of my
texture bitmap than the backside. My aim is to then slowly rotate the cube so as to show its backside.

How do i do that ?
Thanks.

“UV mapping” is the topic you should google.

You should also model your own box. Working with scaled Unity primitives is not going to get you far.