>app/code/vendor/modulename/Setup/UpgradeSchema.php

    <?php
    
    namespace Vendor\Module\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('admin_user');
    
    // Check if the table already exists
    
    if ($setup->getConnection()->isTableExists($tableName) == true) {
    
    // Declare data
    
    $columns = [
    
    'tok_value' => [
    
    'type' => \Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
    
    'nullable' => true,
    
    'comment' => 'Token Value',
    
    ],
    
    ];
    
    $connection = $setup->getConnection();
    
    foreach ($columns as $name => $definition) {
    
    $connection->addColumn($tableName, $name, $definition);
    
    }
    
     
    }
    
    }
    
     
    $setup->endSetup();
    
    }
    
    }



**Note:** If you face any issue, it may be due to the module that you have already installed. As you know, if the module is already installed then **setup:upgrade** command does not install schema. You will need to look into your **setup_module** table, delete your module from the table and re-run the **php bin/magento setup:upgrade** command.