The refinement panel webpart's FilterCategoriesDefinition property contains the XML that specifies the filter categories. If you want to modify the categories, don't forget to clear the "Use Default Configuration" checkbox.
Display all managed metadata fields
Display only specific managed metadata fields
Place the following XSL stylesheet into the XSL property of the refinement panel to view the raw XML, returned by the search...
The raw XML outputs the different filtercategories. Each filtercategory has a specific ID with prefix 'Metadata_xxxxx'. Copy the last part of the ID (i.e. 'xxxxx') and use this value to set the title of the category within the filter category definition section: