Create a new folder called "LiquidCrystal_I2C" under the folder named "libraries" in your Arduino sketchbook folder. Create the folder "libraries" in case it does not exist yet. Place all the files in the "LiquidCrystal_I2C" folder.
To use the library in your own sketch, select it from Sketch > Import Library.
LiquidCrystal Arduino library for the DFRobot I2C LCD displays