Hi MindKill,
What is never obvious is a persons experience/expertise with both FPS Creator and Windows. The answer to your question is in your question. The easiest way to change textures is to change the textures. Find the texture you want to change and either alter it using a graphics program or replace it with a different image file. What we need to know is what part exactly you need help with; finding the texture files and directories in FPSC, opening and altering texture files (.dds), swapping and renaming files in the Program Files directory (permissions), clearing FPSC memory (.dbo and .bin) files after making changes. Once we know where you're at, it'll be simple to help.
Brian.
Characters. You build them a world and what do they do? They moonwalk, get stuck in the walls and fall through the floors!