Update all of the instances of a field in a Drupal 8 database

module_name.install

<?php

function _update_table_field($fieldName, $valueMapping) {
    $schema = \Drupal::database()->schema();
    $tables = $schema->findTables('%');
    foreach ($tables as $table) {
        if ($schema->fieldExists($table, $fieldName)) {
            foreach ($valueMapping as $originalValue => $newValue) {
                echo sprintf(
                    'Updating table %s and field %s from %s to %s',
                    $table,
                    $fieldName,
                    $originalValue,
                    $newValue
                ). PHP_EOL;
                \Drupal::database()
                    ->update($table)
                    ->fields([$fieldName => $newValue])
                    ->condition($fieldName, $originalValue)
                    ->execute();
            }
        }
    }
}


function module_name_update_N(&$sandbox) {
    $fieldName = 'langcode';
    $valueMapping = [
        'en' => 'und',
        'en-GB' => 'en-gb'
    ];
    _update_table_field($fieldName, $valueMapping);
}