What is UpgradeData in Magento2:
We can update the mysql table data with out using magento model/resource model files. This one will helpful to get table row and update data value using upgrade method in upgradeData.php Ex: getTableRow , updateTableRow
Once script is ready then update your module.xml file with new schema & setup version. And then run upgrade command on terminal.
Ex: root> php bin/magento setup:upgrade
CORNER OF BLOG: