As we continually improve Unity, new features are added, existing features are improved - and sometimes old features are removed. With each release, the Manual and Script Reference changes to reflect this, and so you should make sure you are using the correct version of the manual to match the version of Unity you’re using.
The latest version of the documentation is always available online at docs.unity3d.com
In addition to the online manual, the documentation can be installed locally on your computer when you install the Unity software. Prior to Unity 5.3, the documentation was always installed along with the software. From version 5.3 onwards, the Unity Download Assistant allows you to optionally include the documentation for local installation.
For most users, you will be using the latest version of Unity, and therefore the latest version of the documentation.
Some users will need to use older versions of Unity. This might be the case for you if you are maintaining a legacy project, or if you have locked-down to a particular version of Unity during long-term developent of a project.
If you are using an older version of Unity, the locally-installed documentation will match that version of Unity. However if you chose not to install local documentation, older versions of the Unity documentation are available online at the following locations: