Place the install.php file in the same directory as your site root. Load it in your browser and select the version you want to install. (i.e. mydomain.com/install.php)
The script will download the selected version from the MODX respository, unzip it in the same location, then forward you to the MODX setup. The install.php script will automatically delete itself.
This can be used to either upgrade an existing installation or handle a new installation.
In order to be able to directly download the MODX zip archive from the MODX repository, your server must have either allow_url_fopen or cURL enabled.
You may need to change the permissions for newly created folders on line 145. For example, your server may require directory permissions of 0755.