Change Drupal 8 'Powered By' block

You can't change the body of this block fron the user interface. But you can do that using a small custom module using hook_block_view_alter().

Example: in the yourmodule.module

function yourmodule_block_view_alter(array &$build, \Drupal\Core\Block\BlockPluginInterface $block) {
  if ($block->getBaseId() === 'system_powered_by_block') {
    $build['#pre_render'][] = '_yourmodule_block_poweredby_prerender';
  }
}

function _yourmodule_block_poweredby_prerender(array $build) {
    $build['content']['#markup'] = 'Your text';
    return $build;
}

Then clear the cache

NOTE: You can also remove the system 'Powered by Drupal' block and replace by a custom block.