Take a look at: Library management in KiCad version 5
It has a full section for the topic “reset to default” and also details on how to manually add libs (*) if the reset to default workflow seems to dangerous. (It also gives general advice on how to properly setup your personal library system to ensure that it works well.)
(*) TlDr: adding multiple libs using the library manager is done by using shift plus leftclick or crtl plus leftclick to select multiple libs using the file browser like tool that opens for adding libs.