First of all, in order to change textures, press CTRL+M for Map Settings.
Secondly, there is a neat feature in Polyworks called the Texture Quad. Enable it by right-clicking with the Poly Creation tool.
After you have enabled Texture Quad, open the Texture Window, which will display the texture in some sort of grid. On the Texture Window, click-n-hold and select whatever portion of the texture you want to use. Then use the Poly Creation tool.