UpgradeSchema.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Magento\TestSetupDeclarationModule7\Setup;

use Magento\Framework\Setup\InstallSchemaInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\SchemaSetupInterface;
use Magento\Framework\Setup\UpgradeSchemaInterface;

/**
 * UpgradeSchema mock class
 */
class UpgradeSchema implements UpgradeSchemaInterface
{
    /**
     * {@inheritdoc}
     */
    public function upgrade(SchemaSetupInterface $setup, ModuleContextInterface $context)
    {
        $installer = $setup;
        $installer->startSetup();

        if (version_compare($context->getVersion(), '2.0.1') < 0) {
            $installer
                ->getConnection()
                ->modifyColumn('test_table', 'float', ['type' => 'float', 'default' => 29]);
        }

        $installer->endSetup();
    }
}