You actually use the command to install the Xcode command line tools - xcode-select --install . Hence the error message: the tools are already installed.
The command you need to update Xcode is softwareupdate command [args ...] . You can use softwareupdate --list to see what is available, and then softwareupdate --install -a to install all the updates, or softwareupdate --install <product name> to install only the softwareupdate --install <product name> update (if available). You can get the name from the list command.
As mentioned in the comments here, this is a man page for the softwareupdate tool.
2019 update
Many users run into problems when softwareupdate --install -a does not actually softwareupdate --install -a to the latest version of Xcode . The reason for this is most likely the upcoming macOS update (as @brianlmerritt below). In most cases, updating macOS will be the first to solve the problem and also allow you to update Xcode .
Xcode Command Line Tools Update
Most users use this answer when trying to update the Xcode command line tools . The easiest way to achieve this is to uninstall the old version of the tools and install a new one.
sudo rm -rf /Library/Developer/CommandLineTools xcode-select --install
A pop-up window will appear that guides you through the rest of the process.
Vel Genov Jan 05 '16 at 17:38 2016-01-05 17:38
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