If the new textures are in MAT poses you just click and there is the new texture. Make sure you have the horse selected first though. Otherwise its the Materials Room for you. I suggest using the simple version to change the maps.

"Few are agreeable in conversation, because each thinks more of what he intends to say than that of what others are saying, and listens no more when he himself has a chance to speak." - Francois de la Rochefoucauld

Intel Core i7 920, 24GB RAM, GeForce GTX 1050 4GB video, 6TB HDD space
Poser 12: Inches (Poser(PC) user since 1 and the floppies/manual to prove it!)