XPLMSetMenuItemName

XPLM_API void       XPLMSetMenuItemName(
                         XPLMMenuID           inMenu,    
                         int                  inIndex,    
                         const char *         inItemName,    
                         int                  inDeprecatedAndIgnored);    

This routine changes the name of an existing menu item. Pass in the menu ID and the index of the menu item.