After uninstalling the Arduino IDE delete the folder below for a complete uninstall:


On Mac OS X:
/Users/(username)/Library/Arduino15

On Windows (you have to type it in the explorer address bar because its a hidden folder):
Arduino IDE 1.6.5r5 and previous:

C:\Users\(username)\AppData\Roaming\Arduino15


Arduino IDE 1.6.6 and later:
C:\Users\(username)\AppData\Local\Arduino15

On Linux:
/home/(username)/.arduino15    (a.k.a. ~/.arduino15)

If you delete this folder then you will need to reinstall any Boards you have installed using Boards Manager after you reinstall the Arduino IDE.