sabre/xml must be installed using Composer.
You will need a recent version of PHP. sabre/xml 2.0 requires PHP 7, but sabre/xml 1.5, which is still actively maintained, requires PHP 5.5 and up.
Installing with Composer
If Composer is not yet on your system, follow the instructions on getcomposer.org to install it.
From your project directory, the easiest is then to simply call:
composer require sabre/xml
After running this, sabre/xml should be installed, and you can load it in by including the autoloader:
The sabre/xml source can be found on GitHub.