Select from  Menu - File - New library
or use this Quick Button
New library button.

If you have opened library and project, you can access the Export option. Simply press Export button at the lower left corner of New Library panel as shown below:

Export Library.

There are two variants:

Export Not Used Tiles into New Library... - this will asks you for a new library file name and will exports library records that were not used in currently opened and rendered project into the new file. If your project is not rendered, it will exports all library tiles into a new file. If all library tiles were already used for rendering, the new file will be empty.

Export Used Tiles into New Library... - this will asks you for a new library file name and will exports library records that were used in currently opened and rendered project into the new file. If your project is not rendered, it will not exports any tiles and create empty library file. If all library tiles were already used for rendering, the new file will contain all tiles from the current library.