Uninstall Order Number Customiser (M2)

From a command line in the Magento root folder run: 

bin/magento module:disable Fooman_OrderNumberCustomiser
bin/magento setup:upgrade
composer remove --update-with-dependencies fooman/ordernumbercustomiser-m2

If you have used the Manual Installation Method (Option 2), then please delete the following files if they exist.

rm vendor/fooman/packages/Fooman_OrderNumberCustomiser*.zip