The schematic symbol library editor is a tool for editing the schematic symbols in a .lib file. (Remember that .lib files are not the only libraries. Collections of footprints, .kicad_mod files in a XX.pretty folder, or (being deprecated) .mod files are also a libraries. Libraries of footprints.)
There is a button on KiCad's central project manager window to launch the schematic symbol library editor.
(The old (before 4.0.4 or maybe even before that) path of access was from a button on eeSchema's top toolbar.)
The name can be misunderstood. The tool is not for editing a library; it is for editing the schematic symbols held within the library.
See also the PCB footprint editor It is to footprints what the schematic symbol library editor is to schematic symbols.
