Server needs Composer installed (may well already have it).
Log in via SSH
Instructions here: https://github.com/PHPOffice/PhpSpreadsheet
But basically you just use this command: composer require phpoffice/phpspreadsheet
That will do the install.
PhpSpreaqdsheet will get added to the servers root “/vendor” folder, so in “/vendor/phpoffice/phpspreadsheet/”
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.