Change the text field maximum length

module_name.install

<?php

/**
 * Update the length of a text field which already contains data.
 *
 * @param string $entity_type_id
 * @param string $field_name
 * @param integer $new_length
 */
function _module_change_text_field_max_length ($entity_type_id, $field_name, $new_length) {
  $name = 'field.storage.' . $entity_type_id . "." . $field_name;
 
  // Get the current settings
  $result = \Drupal::database()->query(
    'SELECT data FROM {config} WHERE name = :name',
    [':name' => $name]
  )->fetchField();
  $data = unserialize($result);
  $data['settings']['max_length'] = $new_length;
 
  // Write settings back to the database.
  \Drupal::database()->update('config')
    ->fields(array( 'data' => serialize($data)))
    ->condition('name', $name)
    ->execute();
 
  // Update the value column in both the _data and _revision tables for the field
  $table = $entity_type_id . "__" . $field_name;
  $table_revision = $entity_type_id . "_revision__" . $field_name;
  $new_field = ['type' => 'varchar', 'length' => $new_length];
  $col_name = $field_name . '_value';
  \Drupal::database()->schema()->changeField($table, $col_name, $col_name, $new_field);
  \Drupal::database()->schema()->changeField($table_revision, $col_name, $col_name, $new_field);
 
  // Flush the caches.
  drupal_flush_all_caches();
}


function module_name_update_N(&$sandbox) {
  _module_change_text_field_max_length('node', 'field_text', 280);
}

> NOTE: Codee snippet took from http://agaric.com/blogs/change-text-field-maximum-length-drupal-8