Select your source library
2a. Load your desired compenent
If destination lib does not yet exist:
3a. save component in new library
If the destination lib does exist:
4a. Selct your destination library
4b. Update component in library
If you copied all save current library
(otherwise the destination library will not be updated on the disk.)
It would be faster to use a text editor.
Simply copy your .lib file and give it your desired name.
Delete everything within this copy that should not be in this library
Good idea.
Don’t forget the .dcm file if you have aliases or descriptions added to your symbols.
By the way, kicad does not recognize changes done outside of it. If you want to see changes you have made within a text editor you need to close kicad completely and open it again. (At least that is the only way i know how to get kicad to update externally changed symbol libraries.)