Updating the ChromiumOS source tree is done via the repo tool that was installed when you installed the build dependencies earlier in this guide. Assuming you have setup your ChromiumOS development environment according to the instructions in this guide, your ChromiumOS build directory will be located in ~/Workspace/ChromiumOS/ however if you place your ChromiumOS build directory in a different location or on a different disk, modify the commands below to browse to your ChromiumOS build directory.
cd ~/Workspace/ChromiumOS
repo sync
The update process may take several minutes to complete depending on the speed of your internet connection.