If you start with modifying Schematic symbols, you should know a bit about library management.
KiCad’s default libraries are read-only because they are a part of KiCad and can be updated / changed with KiCad updates, and then you would loose your changes if you modified them.
So in short, the order to do things is:
- Open the KiCad Project Manager.
- Open the Schematic Symbol editor.
- Symbol Editor / File / New Library
- Enter a filename. Make it a “Project Specific Library” (for starters).
- Browse to an already existing symbol, right click on it and Save a Copy as.
- Click on your newly created library name.
- Double click on the fresh copy in your new library to load it in the editor.
For a more formal and complete tutorial about library management, have a look at:
The FAQ part of this forum has quite a lot detailed tutorials about different parts of KiCad.