With Reassign Material, a material in the structure is replaced by another material. This means that the material is changed for all Material IDs which are set to this material. In the example below, Glass is changed to Carbon Fiber.

The material to be changed can be selected from the Old Material drop-down menu, which lists all materials that are present in the currently loaded structure.

By clicking on the material name of New Material, the Material Selector opens and the desired new material can be chosen.