I updated the text in the paragraph:
[Note that the genesis files are stored under /var/lib/algorand/genesis/ for DEB and RPM installs] These genesis files can be used to create a new data directory for the specific network. Assume you have installed your node in the ~/node directory and your data directory is ~/node/data . You can switch to TestNet using the following steps [If you are running a DEB/RPM install of the node, you can replace the genesis file in the current data directory which should be /var/lib/algorand ].
So just replace the genesis file with the one for the network you want to run.