To make these and other settings changes while editing a schematic library, Go to Tools ► Document Options which will open the Properties panel where you can change from mils to mm and check the box to Show Comment/Designator.
Here's a link to the relevant section of documentation on the Properties panel with more detailed information on the settings available in this panel: https://www.altium.com/documentation/altium-designer/interactiveproperties-pnl-propertiesproperties-ad#schematic-library-support