How to remove extra column from database table using Magento 2?

In Magento 2 Sometimes we have added an extra column in the table for extended functionality in module and in future, we have removed that functionality so we need to remove extra column field from a database table.

I have added one field name gst in sales_order table and I want to remove extra field from table in future. You can remove field using dropColumn() in UpgradeSchema.php file.
Using below code snippet you can remove already existed column field from table.

Upgrade the setup_version number of module from module.xml file. An older version number is 1.0.1 so I have added new version 1.0.2 in module.xml file

Path: Magento22/app/code/Rbj/Training/etc/module.xml

<?xml version="1.0" ?>
<config xmlns:xsi="" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
	<module name="Rbj_Training" setup_version="1.0.2"/>

Create UpgradeSchema.php file under Setup folder in your module.
Path: Magento22/app/code/Rbj/Training/Setup/UpgradeSchema.php

namespace Rbj\Training\Setup;

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

 * Upgrade the Sales_Order Table to remove extra field
class UpgradeSchema implements UpgradeSchemaInterface

    public function upgrade(SchemaSetupInterface $setup, ModuleContextInterface $context)
        if (version_compare($context->getVersion(), '1.0.2', '<')) {
            $setup->getConnection()->dropColumn($setup->getTable('sales_order'), 'gst');

Run command from root directory of your project,
php bin/magento setup:upgrade

If you want to add the field in table  Add New Field in Database Table Magento 2
If you want to rename/change column field name, Refer link Rename Column name in Table Magento 2