If you are lucky enough to still be using Poser 11, go to the material room on the simple tab. Select a material you want to change with the eye dropper tool. Click the panel displaying the texture and click browse. That should take you to the folder in your DAZ Studio library where the materials for the model are located. Select a different file and execute. If the textures are from a third party, such as a freebie, look at the download file to see where the textures are stored.