Update a basefield definition for an existing entity.

update-entity-field.php

<?php
use Drupal\Core\Field\BaseFieldDefinition;
$definition_manager = \Drupal::entityDefinitionUpdateManager();
$field_storage_definition = $definition_manager->getFieldStorageDefinition('alternate_payment', 'spending_request');
// IF THIS IS UPDATING AN EXISTING FIELD
// $definition_manager->uninstallFieldStorageDefinition($field_storage_definition);
$new_field = BaseFieldDefinition::create('integer')
      ->setLabel(t('Alternate Payment'))
      ->setDescription(t('Track spending requests that do not have internal payments.'))
      ->setDisplayOptions('view', [
        'label' => 'hidden',
        'type' => 'number_integer',
        'weight' => 0,
      ])
      ->setDisplayOptions('form', [
        'type' => 'number',
        'weight' => 0,
      ])
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE);
$definition_manager->installFieldStorageDefinition('alternate_payment', 'spending_request', 'spending_request', $new_field );