The LCD library comes packaged as a whole. The contents of each release can be downloaded from the download section of this wiki.
To instal the library under Arduino's IDE (not for version 1.5):
1. move the contents of the stock LiquidCrystal library from the IDE. This is located in the library folder under your Arduino's distribution.
2. download the latest version of the library from the download section.
3. uncompress the library and copy it to library folder under your Arduino's IDE distribution.
4. Make sure that the folder is named LiquidCrystal library.
To instal the library under Arduino's IDE 1.5:
Follow step 1 and 2
3. move the contents of the documentation folder somewhere for your reference. The LiquidCrystal directory should not have this document folder in it (this will be fixed in future releases).
4. From you Arduino IDE import the library or alternatively proceed to step 3 and 4 previously stated.