You're reading the documentation for a version of ROS 2 that has reached its EOL (end-of-life), and is no longer officially supported. If you want up-to-date information, please have a look at Iron.
Installing the latest ROS 2 development
If you plan to contribute directly to the latest ROS 2 development, you can install ROS 2 by building it from source or installing pre-release binaries. This will give you the latest bug fixes and features.
See Pre-release Testing.
Building from source
The latest development does not go through the same rigorous testing as releases and is not recommended if you are looking for a stable version of ROS 2. Instead, choose a ROS distribution that has already been released.
Follow the links below for the latest setup instructions for your platform:
For keeping your source code up-to-date, see Maintaining a Source Checkout.