Create a .bat to run the cron job with this,
C:\xampp\php\php.exe C:\xampp\htdocs\magento\bin\magento cron:run
C:\xampp\php\php.exe C:\xampp\htdocs\magento\update\cron.php
C:\xampp\php\php.exe C:\xampp\htdocs\magento\bin\magento setup:cron:run
And configure your php.ini with 1024M memory_limit and your .htaccess root file with 1024M as well.
Run below command,
composer require magento/product-community-edition 2.1.12 --no-update
composer update
You can also lookup this below link if above trick is not working for you,
http://devdocs.magento.com/guides/v2.0/release-notes/tech_bull_21-upgrade.html