Fatal error: Out of memory (in composer) during upgrade Magento2.2.1 to Magento 2.3 when run composer update command

Run this command

composer require -dmemory_limit=-1 magento/product-community-edition your update version --no-update

You need to contact the siteground support from your siteground account support section as I had faced same issue and contacted the support they increase the PHP memory limit and issue solved.

1- Locate your main php.ini file by executing php --ini You will find the path in

Loaded Configuration File:         PATHTOphp.ini

2- update memory_limit to a higher value or -1 to maximum

3- save file and rerun your composer command again.

Second Option if you can't edit the main php.ini

You can use custom .ini file to run composer

php -n -c /PATH/TO/custom.ini composer.phar

and download composer.phar from getcomposer website