Some libraries exist in the official repository and you can install them in the following steps:
- If it exists, then just go to Sketch | Include Library | Manage Libraries:
- In a new window, delete the "Filter your search..." and write Arduino Json. The Arduino IDE will then search this library for you and if it has found it, you will be able to install it by clicking on it. You can also use this menu for updating a previously installed library or to change between versions of a library:
- After the library is installed, you will see the following screen:
As an exercise, do the same for WiFiManager and PubSubClient libraries.