Do not modify system libraries. Make your own personal libs as you will loose your changes on update otherwise.
Here some info on how to do this: Creating a new footprint library
In addition: The kicad team does no longer suggest the use of the github plugin. For this reason kicad 5 no longer uses it for its library. If you run v5 then please read this: I had KiCad 4 installed previosly. Now i updated to v5. Now i have some problems with the library setup
For info about installing footprint libs take a look at this: How can i install a specific version of the footprint library?
A more detailed answer to a similar question can be foun here: High level thinking