Magento2 : How to database schema upgrade

If you want to add more column into existing table of your module you could do following.

Step 1: Create UpgradeSchema.php under Setup folder. Get Idea from following code.

namespace Vendor\ModuleName\Setup;

use Magento\Framework\Setup\UpgradeSchemaInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\SchemaSetupInterface;
class UpgradeSchema implements  UpgradeSchemaInterface
{
    public function upgrade(SchemaSetupInterface $setup,
                            ModuleContextInterface $context){
        $setup->startSetup();
        if (version_compare($context->getVersion(), '1.0.1') < 0) {

            // Get module table
            $tableName = $setup->getTable('table_name');

            // Check if the table already exists
            if ($setup->getConnection()->isTableExists($tableName) == true) {
                // Declare data
                $columns = [
                    'imagename' => [
                        'type' => \Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
                        'nullable' => false,
                        'comment' => 'image name',
                    ],
                ];

                $connection = $setup->getConnection();
                foreach ($columns as $name => $definition) {
                    $connection->addColumn($tableName, $name, $definition);
                }

            }
        }

        $setup->endSetup();
    }
}

Step 2: Change the setup_version value in module.xml

Step 3: Run php bin/magento setup:upgrade command from CLI

To upgrade Installer Schema you have to write 'UpgradeSchema.php',

example of UpgradeSchema.php:

namespace <namespace>\<modulename>\Setup;

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

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

    $installer->startSetup();

    /*your code here*/

    $installer->endSetup();
}
}

Step 2: In your module you will find module.xml inside etc folder in that file change setup_version value(ex: 1.0.1 to 1.0.2) version value should be higher then current version value.

Step 3: Run php bin/magento setup:upgrade command from CLI