In Poser, you can't. You can use the group tool to change the mat zones for parts or all of a prop (or figure) and you can also add them there, but you can't remove or rename existing ones. You'll have to use other software for that. Most 3d modelers can do it. I don't know if UVmapper can.

To do this in a 3d modeler, export the obj (or use the original) and open it in the 3d app. Change the zones. export as an obj. Import into poser and save as a new prop.

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